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.h

@@ -28,7 +28,8 @@
 #include "filesystem/File.h"
 #include "video/Video.h"
 #include "thread/threads.h"
-#include "VideoStream.h"
+#include "video/VideoStream.h"
+#include "TheoraVideoStream.h"
 
 namespace love
 {
@@ -63,13 +64,13 @@ public:
 	// Implements Threadable
 	void threadFunction();
 
-	void addStream(VideoStream *stream);
+	void addStream(TheoraVideoStream *stream);
 	// Frees itself!
 	void stop();
 
 private:
 
-	std::vector<StrongRef<VideoStream>> streams;
+	std::vector<StrongRef<TheoraVideoStream>> streams;
 
 	love::thread::MutexRef mutex;
 	love::thread::ConditionalRef cond;