This is a step toward H.265 recording support (#33) by using Retina
for the `VisualSampleEncoding` instead of the prior H.264-specific
logic. The only further change to Moonfire itself needed for H.265 is to
add `| "h265"` to the check for the right video stream.
This also adds support for recording MJPEG, although browser playback
almost certainly won't work.