test out other video decoding backends

[iejMac]

pyAV - untested
VPF - untested

decord:
initial testing showed no clear performance advantage over video2numpy

[guest271314]

I think this can be done in the browser using HTMLMediaElement.seekToNextFrame() https://bugs.chromium.org/p/chromium/issues/detail?id=1065675, and in real-time using MediaStreamTrackProcessor.