Clean up the cursor clipping area when quitting SDL.

This fixes the cursor being clipped after the streaming client quits when streaming Dungeons of Dredmor

src/events/SDL_mouse.c

@@ -362,6 +362,7 @@ SDL_MouseQuit(void)
     SDL_Cursor *cursor, *next;
     SDL_Mouse *mouse = SDL_GetMouse();
 
+    SDL_SetRelativeMouseMode(SDL_FALSE);
     SDL_ShowCursor(1);
 
     cursor = mouse->cursors;