Close SDL2 window when quitting Video subsystem

SDL2 doesn't close windows anymore when the video subsystem is closed.
Therefore, the window created by the SDL2 compat layer has to be closed
manually to avoid orphan windows in frontends like m64py in non-VidExt mode.

src/api/vidext.c

@@ -123,6 +123,9 @@ EXPORT m64p_error CALL VidExt_Quit(void)
         return M64ERR_NOT_INIT;
 
     SDL_ShowCursor(SDL_ENABLE);
+#if SDL_VERSION_ATLEAST(2,0,0)
+    SDL2_DestroyWindow();
+#endif
     SDL_QuitSubSystem(SDL_INIT_VIDEO);
     l_pScreen = NULL;
     l_VideoOutputActive = 0;