Full Theora video support in VideoStreamPlayer

- Implement `set_stream_position` and `get_stream_length`.
- Don't show blank frame when stopping the video (smooth loops).
- Fix audio for videos with up to 8 channels.
- Improve internal audio handling.

scene/gui/video_stream_player.cpp

@@ -339,7 +339,6 @@ void VideoStreamPlayer::play() {
 	if (playback.is_null()) {
 		return;
 	}
-	playback->stop();
 	playback->play();
 	set_process_internal(true);
 	last_audio_time = 0;
@@ -468,7 +467,9 @@ double VideoStreamPlayer::get_stream_position() const {
 
 void VideoStreamPlayer::set_stream_position(double p_position) {
 	if (playback.is_valid()) {
+		resampler.flush();
 		playback->seek(p_position);
+		last_audio_time = 0;
 	}
 }