Platform: hint SDL what GLES library we want to use.

doc/changelog.dox

@@ -46,6 +46,14 @@ See also:
     recognizes also uppercase file extensions (see
     [mosra/magnum#312](https://github.com/mosra/magnum/pull/312))
 
+@subsubsection changelog-latest-changes-platform Platform libraries
+
+-   On OpenGL ES builds, @ref Platform::Sdl2Application now tells SDL whether
+    to use a system GL driver or a dedicated GLES driver based on whether
+    @ref MAGNUM_TARGET_DESKTOP_GLES is defined. This allows for a smoother
+    experience for example when using ANGLE on Windows. See
+    @ref Platform-Sdl2Application-usage-gles for more information.
+
 @subsubsection changelog-latest-changes-trade Trade library
 
 -   The @ref Trade::AnyImageImporter "AnyImageImporter",

src/Magnum/Platform/Sdl2Application.cpp

@@ -97,6 +97,12 @@ Sdl2Application::Sdl2Application(const Arguments& arguments, NoCreateT):
     #ifdef SDL_HINT_NO_SIGNAL_HANDLERS
     SDL_SetHint(SDL_HINT_NO_SIGNAL_HANDLERS, "1");
     #endif
+    /* Available since 2.0.6, uses dedicated OpenGL ES drivers by default and a
+       desktop GLES context only if MAGNUM_TARGET_DESKTOP_GLES is defined as
+       well. */
+    #if !defined(MAGNUM_TARGET_DESKTOP_GLES) && defined(SDL_HINT_OPENGL_ES_DRIVER)
+    SDL_SetHint(SDL_HINT_OPENGL_ES_DRIVER, "1");
+    #endif
     /* Available since 2.0.8, disables compositor bypass on X11, which causes
        flickering on KWin as the compositor gets shut down on every startup */
     #ifdef SDL_HINT_VIDEO_X11_NET_WM_BYPASS_COMPOSITOR

src/Magnum/Platform/Sdl2Application.h

@@ -305,6 +305,17 @@ If you enable @ref Configuration::WindowFlag::Resizable, the canvas will be
 resized when size of the canvas changes and you get @ref viewportEvent(). If
 the flag is not enabled, no canvas resizing is performed.
 
+@subsection Platform-Sdl2Application-usage-gles OpenGL ES specifics
+
+For OpenGL ES, SDL2 defaults to a "desktop GLES" context of the system driver.
+Because Magnum has the opposite default behavior, if @ref MAGNUM_TARGET_GLES is
+not defined and SDL >= 2.0.6 is used, @ref Sdl2Application sets the
+`SDL_HINT_OPENGL_ES_DRIVER` hint to 1, forcing it to load symbols from a
+dedicated libGLES library instead, making SDL and Magnum consistently use the
+same OpenGL entrypoints. This change also allows @ref platforms-gl-es-angle "ANGLE"
+to be used on Windows simply by placing the corresponding `libEGL.dll` and
+`libGLESv2.dll` files next to the application executable.
+
 @section Platform-Sdl2Application-dpi DPI awareness
 
 On displays that match the platform default DPI (96 or 72),