Leak fix
Videoplayer fixes, play/pause & seeking

 - Fix rindex overflow
 - Fix audio sample size bugs (was using sample_fmt and channel count of the decoder, instead of the resampled settings). We didn't notice this bug before, because the OpenAL MovieAudioFactory tries to resample to a format of the same byte size.
 - Add support for play/pause and seeking controls (not used by cutscenes in OpenMW)
 - Closing the video when arriving at the stream end is now handled by the user (we may also want to keep the video open and seek back)