WebMaster: disable audio option (not fully implemented yet)
WebMaster: use frame as prefiew
WebMaster: use ffmpeg instead of mkvmerge to mux result

for better stream selection: mkvmerge leaves an empty video track from
1st encoding pass
WebMaster: workaround: subtitles filter fails to render ass files, use ass filter instead
Merge branch 'master' of github.com:pituz/webm-thread