* Check ffmpeg version instead of checking for presence of BTBN_PATH
* Query ffmpeg version in s6 run script instead of subprocessing in every import
* Define LIBAVFORMAT_VERSION_MAJOR in devcontainer too
* Formatting
* Default ffmpeg version to current btbn version so unit tests pass
* Reduce framerate before downscaling
It is cheaper to drop frames and downscale those that remain than it is
to downscale all frames and then drop some of them. This is achieved
with the filter chain `-cv fps=FPS,scale=W:H`, and perhaps was the
original intention. The plain `-r` and `-s` flags do not execute in
order though - they each put themselves at the *end* of the filterchain,
so `-r FPS -s WxH` actually applies the scale filter first, and then the
rate filter.
This fix can halve the CPU used by the detect ffmpeg process.
* Bring back hard rate limits
