Platform: fix a crash in GlfwApplication::setCursor() on GLFW 3.4.

There's four more new cursors and the _cursors array was too small.

At first I got confused because I thought the assertion on top is done
against the CursorMap, which didn't contain the Hidden cursors. So to
avoid confusing myself again in the future, I moved the assert after the
special cases and made both arrays the same size since it doesn't make
sense to have always-empty fields in there.

Similar change is done in Sdl2Application, and an assert is added to
avoid a nondescript crash if the window is not created yet.

doc/changelog.dox

@@ -308,6 +308,8 @@ See also:
 -   Fixed canvas size setup in @ref Platform::EmscriptenApplication to be the
     same in the @ref Platform::EmscriptenApplication::Configuration::WindowFlag::Contextless "Contextless"
     case as for a WebGL-enabled context
+-   Fixed a crash in @ref Platform::GlfwApplication::setCursor() on the
+    upcoming GLFW 3.4
 -   @ref SceneGraph::BasicMatrixTransformation2D,
     @ref SceneGraph::BasicMatrixTransformation3D,
     @ref SceneGraph::BasicRigidMatrixTransformation2D and

src/Magnum/Platform/GlfwApplication.cpp

@@ -797,7 +797,7 @@ constexpr Int CursorMap[] {
 }
 
 void GlfwApplication::setCursor(Cursor cursor) {
-    CORRADE_INTERNAL_ASSERT(UnsignedInt(cursor) < Containers::arraySize(_cursors));
+    CORRADE_ASSERT(_window, "Platform::GlfwApplication::setCursor(): no window opened", );
 
     _cursor = cursor;
 
@@ -811,6 +811,11 @@ void GlfwApplication::setCursor(Cursor cursor) {
         glfwSetInputMode(_window, GLFW_CURSOR, GLFW_CURSOR_NORMAL);
     }
 
+    /* The second condition could be a static assert but it doesn't let me
+       because "this pointer only accessible in a constexpr function". Thanks
+       for nothing, C++. */
+    CORRADE_INTERNAL_ASSERT(UnsignedInt(cursor) < Containers::arraySize(_cursors) && Containers::arraySize(_cursors) == Containers::arraySize(CursorMap));
+
     if(!_cursors[UnsignedInt(cursor)])
         _cursors[UnsignedInt(cursor)] = glfwCreateStandardCursor(CursorMap[UnsignedInt(cursor)]);
 

src/Magnum/Platform/GlfwApplication.h

@@ -624,7 +624,8 @@ class GlfwApplication {
          * @brief Set cursor type
          * @m_since{2020,06}
          *
-         * Default is @ref Cursor::Arrow.
+         * Expects that a window is already created. Default is
+         * @ref Cursor::Arrow.
          */
         void setCursor(Cursor cursor);
 
@@ -735,7 +736,15 @@ class GlfwApplication {
 
         void setupCallbacks();
 
-        GLFWcursor* _cursors[8]{};
+        /* Corresponds to size of the Cursor enum, the two Hidden cursors are
+           handled differently */
+        GLFWcursor* _cursors[
+            #ifndef GLFW_RESIZE_NWSE_CURSOR
+            6
+            #else
+            10
+            #endif
+        ]{};
         Cursor _cursor = Cursor::Arrow;
 
         /* These are saved from command-line arguments */

src/Magnum/Platform/Sdl2Application.cpp

@@ -1052,7 +1052,7 @@ constexpr const char* CursorMap[] {
 
 void Sdl2Application::setCursor(Cursor cursor) {
     #ifndef CORRADE_TARGET_EMSCRIPTEN
-    CORRADE_INTERNAL_ASSERT(UnsignedInt(cursor) < Containers::arraySize(_cursors));
+    CORRADE_ASSERT(_window, "Platform::Sdl2Application::setCursor(): no window opened", );
 
     if(cursor == Cursor::Hidden) {
         SDL_ShowCursor(SDL_DISABLE);
@@ -1069,11 +1069,17 @@ void Sdl2Application::setCursor(Cursor cursor) {
         SDL_SetRelativeMouseMode(SDL_FALSE);
     }
 
+    /* The second condition could be a static assert but it doesn't let me
+       because "this pointer only accessible in a constexpr function". Thanks
+       for nothing, C++. */
+    CORRADE_INTERNAL_ASSERT(UnsignedInt(cursor) < Containers::arraySize(_cursors) && Containers::arraySize(_cursors) == Containers::arraySize(CursorMap));
+
     if(!_cursors[UnsignedInt(cursor)])
         _cursors[UnsignedInt(cursor)] = SDL_CreateSystemCursor(CursorMap[UnsignedInt(cursor)]);
 
     SDL_SetCursor(_cursors[UnsignedInt(cursor)]);
     #else
+    CORRADE_ASSERT(_surface, "Platform::Sdl2Application::setCursor(): no window opened", );
     _cursor = cursor;
     CORRADE_INTERNAL_ASSERT(UnsignedInt(cursor) < Containers::arraySize(CursorMap));
     #pragma GCC diagnostic push

src/Magnum/Platform/Sdl2Application.h

@@ -968,7 +968,8 @@ class Sdl2Application {
          * @brief Set cursor type
          * @m_since{2020,06}
          *
-         * Default is @ref Cursor::Arrow.
+         * Expects that a window is already created. Default is
+         * @ref Cursor::Arrow.
          */
         void setCursor(Cursor cursor);
 
@@ -1184,7 +1185,7 @@ class Sdl2Application {
         CORRADE_ENUMSET_FRIEND_OPERATORS(Flags)
 
         #ifndef CORRADE_TARGET_EMSCRIPTEN
-        SDL_Cursor* _cursors[14]{};
+        SDL_Cursor* _cursors[12]{};
         #else
         Cursor _cursor;
         #endif