Fixes and Improvements

Thanks to help by Vlad from MagicIndie, I've been able to solve 2 major issues that plagued this library:

1) Alpha decoding problem: Sometimes the alpha channel wasn't decoded properly. Completely random! The cause was an uninitialized variable in TheoraVideoClip (facepalm :)

2) Worker thread scheduling: The previous dumb implementation has now been replaced by a smarter implementation that takes into account user priorities, queued playback time and playback state, ensuring more allocated CPU time where it counts.