SWORD25: Draw next movie frame only when an update is needed

This fixes the problem that the movies are played too fast
and get out of sync with additionally played speech samples.

engines/sword25/fmv/movieplayer.cpp

@@ -123,7 +123,7 @@ void MoviePlayer::update() {
 		if (_decoder.endOfVideo()) {
 			// Movie complete, so unload the movie
 			unloadMovie();
-		} else {
+		} else if (_decoder.needsUpdate()) {
 			const Graphics::Surface *s = _decoder.decodeNextFrame();
 			if (s) {
 				// Transfer the next frame