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/sdl2_v.cpp

@@ -761,6 +761,7 @@ bool VideoDriver_SDL_Base::ToggleFullscreen(bool fullscreen)
 		DEBUG(driver, 0, "SDL_SetWindowFullscreen() failed: %s", SDL_GetError());
 	}
 
+	InvalidateWindowClassesData(WC_GAME_OPTIONS, 3);
 	return ret == 0;
 }