Split theora::VideoStream into OggDemuxer and TheoraVideoStream

This makes the internal API more modular, but the split also forces a cleaner
implementation. It also means the OggDemuxer is now responsible for the
ogg-specific stuff like seeking and (ogg-specific) type detection.

Backported from the love.video-vpx branch of love-experiments

--HG--
branch : minor

src/modules/video/theora/Video.cpp

@@ -46,7 +46,7 @@ Video::~Video()
 
 VideoStream *Video::newVideoStream(love::filesystem::File *file)
 {
-	VideoStream *stream = new VideoStream(file);
+	TheoraVideoStream *stream = new TheoraVideoStream(file);
 	workerThread->addStream(stream);
 	return stream;
 }
@@ -67,7 +67,7 @@ Worker::~Worker()
 	stop();
 }
 
-void Worker::addStream(VideoStream *stream)
+void Worker::addStream(TheoraVideoStream *stream)
 {
 	love::thread::Lock l(mutex);
 	streams.push_back(stream);
@@ -110,7 +110,7 @@ void Worker::threadFunction()
 
 		for (auto it = streams.begin(); it != streams.end(); ++it)
 		{
-			VideoStream *stream = *it;
+			TheoraVideoStream *stream = *it;
 			if (stream->getReferenceCount() == 1)
 			{
 				// We're the only ones left