This is an attempt to improve video performance when encoding
does not keep up with frames. This situation can be reproduced
by running multiple encoders at the same time.
Added `utils/video_stress.js` to manually reproduce this issue.
Observing ffmpeg logs, it does not do any encoding initially and
instead does "input analysis / probing" that detects fps and other
parameters. By the time it starts encoding (launches vpx and creates
the video file), we already have many frames in the buffer.
Reducing probing helps:
`-avioflags direct -fpsprobesize 0 -probesize 32 -analyzeduration 0`
Another issue observed is questionable default `-threads` value.
We compile without threads support, so logs say "using emulated threads".
For some reason, setting explicit `-threads 1` (or any other value)
makes it better when cpu is loaded.
