Fix #8784: using alt+enter didn't update the fullscreen toggle visibly (#8820)

Basically, the window was not invalidated, so it was never redrawn.
This made it look like it wasn't working, but it really was.

src/video/sdl_v.cpp

@@ -763,6 +763,7 @@ bool VideoDriver_SDL::ToggleFullscreen(bool fullscreen)
 		_fullscreen ^= true;
 	}
 
+	InvalidateWindowClassesData(WC_GAME_OPTIONS, 3);
 	return ret;
 }