Platform: remove 50 000 lines from Sdl2Application header.

Of all the libraries I use, I expected SDL to do the "include the world"
horror the least of all. Ugh.

src/Magnum/Platform/Sdl2Application.cpp

@@ -26,6 +26,7 @@
 #include "Sdl2Application.h"
 
 #include <cstring>
+#include <SDL.h>
 #ifndef CORRADE_TARGET_EMSCRIPTEN
 #include <tuple>
 #else

src/Magnum/Platform/Sdl2Application.h

@@ -46,7 +46,10 @@
 #ifdef CORRADE_TARGET_WINDOWS /* Windows version of SDL2 redefines main(), we don't want that */
 #define SDL_MAIN_HANDLED
 #endif
-#include <SDL.h>
+/* SDL.h includes the world, adding 50k LOC. We don't want that either. */
+#include <SDL_keycode.h>
+#include <SDL_mouse.h>
+#include <SDL_video.h>
 #include <SDL_scancode.h>
 
 #ifdef CORRADE_TARGET_WINDOWS_RT