Added new CacheDisk class, which caches frames to the hard drive, dramatically speeding up preview speeds, at the expense of IO operations. New unittests for caching framework. Fixed a few bugs with Frame constructor, which was causing invalid # width & height. Integrated JSON into the cache framework, to quickly share the state of the cache (including ranges of cached frame numbers). Fixed a bug where some Timeline frames could have no audio samples.

src/Qt/VideoCacheThread.cpp

@@ -56,13 +56,13 @@ namespace openshot
     	current_display_frame = current_frame_number;
     }
 
-	// Seek the audio thread
+	// Seek the reader to a particular frame number
 	void VideoCacheThread::Seek(int new_position)
 	{
 		position = new_position;
 	}
 
-	// Play the audio
+	// Play the video
 	void VideoCacheThread::Play() {
 		// Start playing
 		is_playing = true;