internal filters: enable avisynth support
Merge pull request #132 from m0vie/avisynth

Use LAV Splitter by default to demux AviSynth script.

This avoids using the old VFW interface that is said to be less stable than directly using AviSynth through ffmpeg. Note that this means AviSynth is still required to demux the scripts.
PGS Subtitles: Fix a crash in some cases.

Mimic the change for DVB subtitles just for consistency.