N3DS port (squashed)

A dedicated renderer using Citro3D would likely allow for better
much better graphical performances.

include/SDL_video.h

@@ -126,6 +126,7 @@ typedef enum
     SDL_WINDOW_KEYBOARD_GRABBED = 0x00100000,   /**< window has grabbed keyboard input */
     SDL_WINDOW_VULKAN           = 0x10000000,   /**< window usable for Vulkan surface */
     SDL_WINDOW_METAL            = 0x20000000,   /**< window usable for Metal view */
+    SDL_WINDOW_N3DS_BOTTOM      = 0x40000000,   /**< window should be on the bottom screen (N3DS only) */
 
     SDL_WINDOW_INPUT_GRABBED = SDL_WINDOW_MOUSE_GRABBED /**< equivalent to SDL_WINDOW_MOUSE_GRABBED for compatibility */
 } SDL_WindowFlags;