Partially fix data race issues around _cur_palette

See: https://github.com/OpenTTD/OpenTTD/issues/8712

src/video/sdl2_v.cpp

@@ -257,6 +257,7 @@ void VideoDriver_SDL_Base::CheckPaletteAnim()
 	if (_cur_palette.count_dirty == 0) return;
 
 	this->local_palette = _cur_palette;
+	_cur_palette.count_dirty = 0;
 	this->MakeDirty(0, 0, _screen.width, _screen.height);
 }