Platform: set SDL_HINT_OPENGL_ES_DRIVER only on GLES.

I was setting it on desktop GL as well, and *somehow* it worked. That
does not mean it's correct.

src/Magnum/Platform/Sdl2Application.cpp

@@ -142,7 +142,7 @@ Sdl2Application::Sdl2Application(const Arguments& arguments, NoCreateT):
     /* 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)
+    #if defined(MAGNUM_TARGET_GLES) && !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