Platform: don't disable compositing in Sdl2App.

Yay, it was fixed in the meantime!

doc/changelog.dox

@@ -267,6 +267,9 @@ See also:
     @cpp {640, 480} @ce
 -   @ref Platform::Sdl2Application::Configuration::WindowFlags values that make
     no sense on Emscripten are not available there anymore
+-   When using SDL 2.0.8 and newer, @ref Platform::Sdl2Application no longer
+    attempts to disable compositing on Linux. See @ref Platform-Sdl2Application-usage-linux
+    for more information.
 
 @subsubsection changelog-latest-changes-shaders Shaders library
 

src/Magnum/Platform/Sdl2Application.cpp

@@ -95,6 +95,10 @@ Sdl2Application::Sdl2Application(const Arguments& arguments, NoCreateT):
         std::exit(1);
     }
 
+    #ifdef SDL_HINT_VIDEO_X11_NET_WM_BYPASS_COMPOSITOR /* Available since 2.0.8 */
+    SDL_SetHint(SDL_HINT_VIDEO_X11_NET_WM_BYPASS_COMPOSITOR, "0");
+    #endif
+
     /* Save command-line arguments */
     if(args.value("log") == "verbose") _verboseLog = true;
     const std::string dpiScaling = args.value("dpi-scaling");

src/Magnum/Platform/Sdl2Application.h

@@ -228,6 +228,21 @@ If no other application header is included, this class is also aliased to
 @cpp Platform::Application @ce and the macro is aliased to @cpp MAGNUM_APPLICATION_MAIN() @ce
 to simplify porting.
 
+@subsection Platform-Sdl2Application-usage-linux Linux specifics
+
+SDL by default attempts to disable compositing, which may cause ugly flickering
+for non-fullscreen apps (KWin, among others, is known to respect this setting).
+When using SDL >= 2.0.8, @ref Sdl2Application turns this behavior off, keeping
+the compositor running to avoid the flicker. You can turn this behavior back on
+by enabling the [corresponding SDL hint](https://wiki.libsdl.org/CategoryHints) through an environment variable:
+
+@code{.sh}
+SDL_VIDEO_X11_NET_WM_BYPASS_COMPOSITOR=1 ./your-app
+@endcode
+
+If you're running an older version of SDL, you can disallow apps from bypassing
+the compositor in system-wide KWin settings.
+
 @subsection Platform-Sdl2Application-usage-ios iOS specifics
 
 Leaving a default (zero) window size in @ref Configuration will cause the app