Platform: deprecate Sdl2Application MultiGestureEvent.

The more I looked at it, the more useless it felt. Also, since it's so
broken, I'm also completely removing it from the test because there's no
point in even testing it.

doc/changelog.dox

@@ -1437,6 +1437,14 @@ See also:
     @ref Platform::GlfwApplication and the
     @ref Platform::BasicScreenedApplication / @ref Platform::BasicScreen
     wrappers as well.
+-   @cpp Platform::Sdl2Application::multiGestureEvent() @ce and the
+    @cpp MultiGestureEvent @ce event class is deprecated in favor of a new
+    @ref Platform::TwoFingerGesture helper. The SDL event reported relative
+    distance to previous finger positions instead of a relative diameter
+    between multiple pressed fingers, making it practically useless for
+    implementing pinch zoom. It additionally reported everything in normalized
+    coordinates, thus the distance and rotation was only meaningful when the
+    window was an exact square.
 -   @cpp Platform::Sdl2Application::InputEvent::Modifier @ce,
     @cpp Modifiers @ce and @cpp Platform::Sdl2Application::KeyEvent::Key @ce
     enums and the @cpp Platform::Sdl2Application::KeyEvent::keyName(Key) @ce

src/Magnum/Platform/Sdl2Application.cpp

@@ -1212,11 +1212,15 @@ bool Sdl2Application::mainLoopIteration() {
             }
             #endif
 
+            #ifdef MAGNUM_BUILD_DEPRECATED
             case SDL_MULTIGESTURE: {
+                CORRADE_IGNORE_DEPRECATED_PUSH
                 MultiGestureEvent e{event, {event.mgesture.x, event.mgesture.y}, event.mgesture.dTheta, event.mgesture.dDist, event.mgesture.numFingers};
                 multiGestureEvent(e);
+                CORRADE_IGNORE_DEPRECATED_POP
                 break;
             }
+            #endif
 
             case SDL_TEXTINPUT: {
                 TextInputEvent e{event, event.text.text};
@@ -1590,10 +1594,10 @@ void Sdl2Application::scrollEvent(ScrollEvent& event) {
 #ifdef MAGNUM_BUILD_DEPRECATED
 CORRADE_IGNORE_DEPRECATED_PUSH
 void Sdl2Application::mouseScrollEvent(MouseScrollEvent&) {}
+void Sdl2Application::multiGestureEvent(MultiGestureEvent&) {}
 CORRADE_IGNORE_DEPRECATED_POP
 #endif
 
-void Sdl2Application::multiGestureEvent(MultiGestureEvent&) {}
 void Sdl2Application::textInputEvent(TextInputEvent&) {}
 void Sdl2Application::textEditingEvent(TextEditingEvent&) {}
 

src/Magnum/Platform/Sdl2Application.h

@@ -560,8 +560,8 @@ class Sdl2Application {
         class MouseEvent;
         class MouseMoveEvent;
         class MouseScrollEvent;
-        #endif
         class MultiGestureEvent;
+        #endif
         class TextInputEvent;
         class TextEditingEvent;
 
@@ -1353,16 +1353,24 @@ class Sdl2Application {
          * Default implementation does nothing.
          */
         virtual CORRADE_DEPRECATED("use scrollEvent() instead") void mouseScrollEvent(MouseScrollEvent& event);
-        #endif
 
         /**
          * @brief Multi gesture event
+         * @m_deprecated_since_latest The SDL event reports relative distance
+         *      to previous finger positions instead of a relative diameter
+         *      between multiple pressed fingers, making it practically useless
+         *      for implementing pinch zoom. It additionally reports everything
+         *      in normalized coordinates, thus the distance and rotation is
+         *      only meaningful when the window is an exact square. Use the
+         *      @ref TwoFingerGesture helper instead and feed it with events
+         *      coming from @ref pointerPressEvent(),
+         *      @ref pointerReleaseEvent() and @ref pointerMoveEvent().
          *
          * Called when the user performs a gesture using multiple fingers.
          * Default implementation does nothing.
-         * @experimental
          */
-        virtual void multiGestureEvent(MultiGestureEvent& event);
+        virtual CORRADE_DEPRECATED("use pointerEvent(), pointerMoveEvent() and TwoFingerGesture instead") void multiGestureEvent(MultiGestureEvent& event);
+        #endif
 
         /* Since 1.8.17, the original short-hand group closing doesn't work
            anymore. FFS. */
@@ -3401,15 +3409,21 @@ class CORRADE_DEPRECATED("use ScrollEvent and scrollEvent() instead") Sdl2Applic
         Containers::Optional<Vector2i> _position;
         Containers::Optional<Sdl2Application::Modifiers> _modifiers;
 };
-#endif
 
 /**
 @brief Multi gesture event
+@m_deprecated_since_latest The SDL event reports relative distance to previous
+    finger positions instead of a relative diameter between multiple pressed
+    fingers, making it practically useless for implementing pinch zoom. It
+    additionally reports everything in normalized coordinates, thus the
+    distance and rotation is only meaningful when the window is an exact
+    square. Use the @ref TwoFingerGesture helper instead and feed it with
+    events coming from @ref pointerPressEvent(), @ref pointerReleaseEvent() and
+    @ref pointerMoveEvent().
 
-@experimental
 @see @ref multiGestureEvent()
 */
-class Sdl2Application::MultiGestureEvent {
+class CORRADE_DEPRECATED("use TwoFingerGesture with pointerPressEvent(), pointerReleaseEvent() and pointerMoveEvent() instead") Sdl2Application::MultiGestureEvent {
     public:
         /** @brief Copying is not allowed */
         MultiGestureEvent(const MultiGestureEvent&) = delete;
@@ -3480,6 +3494,7 @@ class Sdl2Application::MultiGestureEvent {
         const Int _fingerCount;
         bool _accepted;
 };
+#endif
 
 /**
 @brief Text input event

src/Magnum/Platform/Test/Sdl2ApplicationTest.cpp

@@ -344,13 +344,9 @@ struct Sdl2ApplicationTest: Platform::Application {
     #if 1
     void pointerPressEvent(PointerEvent& event) override {
         Debug{} << "pointer press:" << event.source() << event.pointer() << (event.isPrimary() ? "primary" : "secondary") << event.id() << event.modifiers() << Debug::packed << event.position();
-        _gestureDistance = {};
-        _gestureRotation = {};
     }
     void pointerReleaseEvent(PointerEvent& event) override {
         Debug{} << "pointer release:" << event.source() << event.pointer() << (event.isPrimary() ? "primary" : "secondary") << event.id() << event.modifiers() << Debug::packed << event.position();
-        _gestureDistance = {};
-        _gestureRotation = {};
     }
     void pointerMoveEvent(PointerMoveEvent& event) override {
         Debug{} << "pointer move:" << event.source() << event.pointer() << event.pointers() << (event.isPrimary() ? "primary" : "secondary") << event.id() << event.modifiers() << Debug::packed << event.position() << Debug::packed << event.relativePosition();
@@ -362,13 +358,9 @@ struct Sdl2ApplicationTest: Platform::Application {
     CORRADE_IGNORE_DEPRECATED_PUSH
     void mousePressEvent(MouseEvent& event) override {
         Debug{} << "mouse press:" << event.button() << Debug::packed << event.position() << event.modifiers();
-        _gestureDistance = {};
-        _gestureRotation = {};
     }
     void mouseReleaseEvent(MouseEvent& event) override {
         Debug{} << "mouse release:" << event.button() << Debug::packed << event.position() << event.modifiers();
-        _gestureDistance = {};
-        _gestureRotation = {};
     }
     void mouseMoveEvent(MouseMoveEvent& event) override {
         Debug{} << "mouse move:" << event.buttons() << Debug::packed << event.position() << Debug::packed << event.relativePosition() << event.modifiers();
@@ -379,12 +371,6 @@ struct Sdl2ApplicationTest: Platform::Application {
     CORRADE_IGNORE_DEPRECATED_POP
     #endif
 
-    void multiGestureEvent(MultiGestureEvent& event) override {
-        Debug{} << "multi gesture:" << event.center() << "rotation:" << Deg{_gestureRotation} << Debug::nospace << ", relative" << Deg{event.relativeRotation()} << Debug::nospace << ", distance" << _gestureDistance << Debug::nospace << ", relative" << event.relativeDistance() << Debug::nospace << ", fingers" << event.fingerCount();
-        _gestureDistance += event.relativeDistance();
-        _gestureRotation += Rad(event.relativeRotation());
-    }
-
     void keyPressEvent(KeyEvent& event) override {
         Debug{} << "key press:" << event.key() << event.keyName() << "scancode:" << event.scanCode()
             #ifndef CORRADE_TARGET_EMSCRIPTEN
@@ -517,8 +503,6 @@ struct Sdl2ApplicationTest: Platform::Application {
         #ifndef CORRADE_TARGET_EMSCRIPTEN
         bool _vsync = false;
         #endif
-        Float _gestureDistance{};
-        Rad _gestureRotation{};
 };
 
 Sdl2ApplicationTest::Sdl2ApplicationTest(const Arguments& arguments): Platform::Application{arguments, NoCreate} {