Platform: print the "HiDPI not enabled" warning at most once on macOS.

If the application is constructed with delayed window creation, the
warning would be printed on each context creation attempt / call to
dpiScaling(const Configuration&), which is silly.

src/Magnum/Platform/GlfwApplication.cpp

@@ -54,7 +54,10 @@ static_assert(GLFW_TRUE == true && GLFW_FALSE == false, "GLFW does not have sane
 
 enum class GlfwApplication::Flag: UnsignedByte {
     Redraw = 1 << 0,
-    TextInputActive = 1 << 1
+    TextInputActive = 1 << 1,
+    #ifdef CORRADE_TARGET_APPLE
+    HiDpiWarningPrinted = 1 << 2
+    #endif
 };
 
 GlfwApplication::GlfwApplication(const Arguments& arguments): GlfwApplication{arguments, Configuration{}} {}
@@ -127,14 +130,16 @@ void GlfwApplication::create(const Configuration& configuration, const GLConfigu
 }
 #endif
 
-Vector2 GlfwApplication::dpiScaling(const Configuration& configuration) const {
+Vector2 GlfwApplication::dpiScaling(const Configuration& configuration) {
     std::ostream* verbose = _verboseLog ? Debug::output() : nullptr;
 
     /* Print a helpful warning in case some extra steps are needed for HiDPI
        support */
     #ifdef CORRADE_TARGET_APPLE
-    if(!Implementation::isAppleBundleHiDpiEnabled())
+    if(!Implementation::isAppleBundleHiDpiEnabled() && !(_flags & Flag::HiDpiWarningPrinted)) {
         Warning{} << "Platform::GlfwApplication: warning: the executable is not a HiDPI-enabled app bundle";
+        _flags |= Flag::HiDpiWarningPrinted;
+    }
     #elif defined(CORRADE_TARGET_WINDOWS)
     /** @todo */
     #endif

src/Magnum/Platform/GlfwApplication.h

@@ -428,7 +428,7 @@ class GlfwApplication {
          *
          * How the content should be scaled relative to system defaults for
          * given @ref windowSize(). If a window is not created yet, returns
-         * zero vector, use @ref dpiScaling(const Configuration&) const for
+         * zero vector, use @ref dpiScaling(const Configuration&) for
          * calculating a value independently. See @ref Platform-GlfwApplication-dpi
          * for more information.
          * @see @ref framebufferSize()
@@ -443,7 +443,7 @@ class GlfwApplication {
          * and custom scaling specified on the command-line. See
          * @ref Platform-GlfwApplication-dpi for more information.
          */
-        Vector2 dpiScaling(const Configuration& configuration) const;
+        Vector2 dpiScaling(const Configuration& configuration);
 
         /**
          * @brief Set window title

src/Magnum/Platform/Sdl2Application.cpp

@@ -87,7 +87,10 @@ enum class Sdl2Application::Flag: UnsignedByte {
     Exit = 1 << 4,
     #ifdef CORRADE_TARGET_EMSCRIPTEN
     TextInputActive = 1 << 5,
-    Resizable = 1 << 6
+    Resizable = 1 << 6,
+    #endif
+    #ifdef CORRADE_TARGET_APPLE
+    HiDpiWarningPrinted = 1 << 7
     #endif
 };
 
@@ -180,16 +183,18 @@ void Sdl2Application::create(const Configuration& configuration, const GLConfigu
 }
 #endif
 
-Vector2 Sdl2Application::dpiScaling(const Configuration& configuration) const {
+Vector2 Sdl2Application::dpiScaling(const Configuration& configuration) {
     std::ostream* verbose = _verboseLog ? Debug::output() : nullptr;
 
     /* Print a helpful warning in case some extra steps are needed for HiDPI
        support */
     #ifdef CORRADE_TARGET_APPLE
-    if(!Implementation::isAppleBundleHiDpiEnabled())
+    if(!Implementation::isAppleBundleHiDpiEnabled() && !(_flags & Flag::HiDpiWarningPrinted)) {
         Warning{} << "Platform::Sdl2Application: warning: the executable is not a HiDPI-enabled app bundle";
+        _flags |= Flag::HiDpiWarningPrinted;
+    }
     #elif defined(CORRADE_TARGET_WINDOWS)
-    /** @todo */
+    /* Handled below, warning printed only when using virtual DPI scaling */
     #endif
 
     /* Use values from the configuration only if not overriden on command line.

src/Magnum/Platform/Sdl2Application.h

@@ -730,7 +730,7 @@ class Sdl2Application {
          *
          * How the content should be scaled relative to system defaults for
          * given @ref windowSize(). If a window is not created yet, returns
-         * zero vector, use @ref dpiScaling(const Configuration&) const for
+         * zero vector, use @ref dpiScaling(const Configuration&) for
          * calculating a value independently. See @ref Platform-Sdl2Application-dpi
          * for more information.
          * @see @ref framebufferSize()
@@ -745,7 +745,7 @@ class Sdl2Application {
          * and custom scaling specified on the command-line. See
          * @ref Platform-Sdl2Application-dpi for more information.
          */
-        Vector2 dpiScaling(const Configuration& configuration) const;
+        Vector2 dpiScaling(const Configuration& configuration);
 
         /**
          * @brief Set window title