Platform: SDL disables power-saving when running. That's stupid, don't.

I'm getting kinda pissed at the strange defaults. Should I be using a
different toolkit altogether because SDL IS FOR GAMES ONLY? Given how
many bugreports and complaints there is about "Dosbox blocking
screensaver" I'm beginning to think it's SDL's fault, not mine.

Let the users control what they want, ffs, don't enable problematic
features like blocking powersave or disabling compositor by default.
Those should be a runtime option anyway, similarly to how video players
block powersaving only when *an actual video is playing*.

doc/changelog.dox

@@ -505,6 +505,10 @@ See also:
     @relativeref{Platform::Sdl2Application::KeyEvent::Key,Menu} for consistency
     with @ref Platform::EmscriptenApplication and
     @ref Platform::GlfwApplication (see [mosra/magnum#547](https://github.com/mosra/magnum/pull/547))
+-   @ref Platform::Sdl2Application now overrides SDL's default behavior that
+    prevents computer from entering a power-saving mode while the application
+    is running, as that's generally undesirable for regular GUI applications.
+    See @ref Platform-Sdl2Application-usage-power for more information.
 
 @subsubsection changelog-latest-changes-scenegraph SceneGraph library
 

src/Magnum/Platform/Sdl2Application.cpp

@@ -149,6 +149,14 @@ Sdl2Application::Sdl2Application(const Arguments& arguments, NoCreateT):
     #ifdef SDL_HINT_VIDEO_X11_NET_WM_BYPASS_COMPOSITOR
     SDL_SetHint(SDL_HINT_VIDEO_X11_NET_WM_BYPASS_COMPOSITOR, "0");
     #endif
+    /* By default, SDL behaves like if it was playing a video or whatever,
+       preventing the computer from turning off the screen or going to sleep.
+       While it sorta makes sense for games, it's useless and annoying for
+       regular apps. Together with the compositor disabling those two are the
+       most stupid defaults. */
+    #ifdef SDL_HINT_VIDEO_ALLOW_SCREENSAVER
+    SDL_SetHint(SDL_HINT_VIDEO_ALLOW_SCREENSAVER, "1");
+    #endif
     /* Available since 2.0.12, use EGL if desired */
     #if defined(MAGNUM_TARGET_EGL) && defined(SDL_HINT_VIDEO_X11_FORCE_EGL)
     SDL_SetHint(SDL_HINT_VIDEO_X11_FORCE_EGL, "1");

src/Magnum/Platform/Sdl2Application.h

@@ -286,6 +286,19 @@ 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-power Power management
+
+SDL by default prevents the computer from powering off the or screen going to
+sleep. While possibly useful for game-like use cases, it's generally
+undesirable for regular applications. @ref Sdl2Application turns this behavior
+off. You can restore SDL's default behavior by disabling the
+[corresponding SDL hint](https://wiki.libsdl.org/CategoryHints) through an
+environment variable or through @cpp SDL_SetHint() @ce from your application.
+
+@code{.sh}
+SDL_VIDEO_ALLOW_SCREENSAVER=0 ./your-app
+@endcode
+
 @subsection Platform-Sdl2Application-usage-posix POSIX specifics
 
 On POSIX systems, SDL by default intercepts the `SIGTERM` signal and generates
@@ -314,7 +327,8 @@ 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:
+through an environment variable or through @cpp SDL_SetHint() @ce from your
+application.
 
 @code{.sh}
 SDL_VIDEO_X11_NET_WM_BYPASS_COMPOSITOR=1 ./your-app