Alpha support

[Ironclad17]

          LAV Video does not output any alpha information, and as mentioned several posts above, its not really planned to start doing that either.

Originally posted by @Nevcairiel in #90 (comment)

Just wanted to check if this was still the case. I was converting some gifs to vp9 webms when I noticed transparency wasn't displaying despite being correctly detected by ffprobe. Simply including one of the libavcodec libraries and using libvpx for decode resolves the issue.
https://trac.ffmpeg.org/ticket/8344
ffplay -vcodec libvpx-vp9 "yuva420p.webm
Displays transparency correctly.