Wrapper element for AviSynth filters

by LRN for GStreamer

The goal is to make some AviSynth video filters available as GStreamer pipeline elements. This would also bring AviSynth v3.0 vapor-ware closer to reality. The project consists of: 1) Implementing frame number information extraction in demuxers 2) Implementing frame-based seeking 3) Implementing a video ringbuffer 4) Implementing GstAVSynth wrapper library 5) Implementing GstAVSynth headers/libraries for AviSynth filters to build against