videolang/video: v0.2.1

This is a minor release. There are no major changes, rather, several bugs have been fixed, and a few new features have been added.

A more complete log of changes:

• Fix bug allowing for transparent picture overlays.
• Fix bug preventing picts from being used in multitrack.
• Add highpass/lowpass filters
• Fix color filters when written out to mp4 files
• Fix bug in composite-merge where rgb was separated into different videos.
• Update live-capture (internal) data structure to use device indexes as well as strings.
• Enable ffmpeg logging at the racket layer IF the FF_LOG environment variable is set to racket.
• Add locks to fix potential inconsistent state in ffmpeg.
• Add a 'mirror' option to the renderer, to output to multiple places at once.
• Renderer can now split output into two files one for video, and one for audio.
• Preview window works again with live webcams