Platform: fix Sdl2Application::setSwapInterval(0) + setMinLoopPeriod().

It thought VSync was enabled even when it was set to 0. Also clarified
the docs a bit.

doc/changelog.dox

@@ -185,6 +185,9 @@ See also:
     @cpp #include <SDL_main.h> @ce, causing iOS builds to fail to initialize.
     The include got added back and initialization failures are reported with
     more information now.
+-   Calling @ref Platform::Sdl2Application::setSwapInterval() with @cpp 0 @ce
+    caused @ref Platform::Sdl2Application::setMinimalLoopPeriod() "setMinimalLoopPeriod()"
+    to be ignored even though Vsync was in fact not enabled.
 
 @subsection changelog-latest-deprecated Deprecated APIs
 

src/Magnum/Platform/Sdl2Application.cpp

@@ -719,7 +719,8 @@ bool Sdl2Application::setSwapInterval(const Int interval) {
         return false;
     }
 
-    _flags |= Flag::VSyncEnabled;
+    if(interval) _flags |= Flag::VSyncEnabled;
+    else _flags &= ~Flag::VSyncEnabled;
     return true;
 }
 

src/Magnum/Platform/Sdl2Application.h

@@ -822,7 +822,8 @@ class Sdl2Application {
          *
          * This setting reduces the main loop frequency in case VSync is
          * not/cannot be enabled or no drawing is done. Default is @cpp 0 @ce
-         * (i.e. looping at maximum frequency).
+         * (i.e. looping at maximum frequency). If the application is drawing
+         * on the screen and VSync is enabled, this setting is ignored.
          * @note Not available in @ref CORRADE_TARGET_EMSCRIPTEN "Emscripten",
          *      the browser is managing the frequency instead.
          * @see @ref setSwapInterval()