Found reason - it's due to 'format=gray' before [new] pad. After I
removed that conversion - it's again at full fps even with all ffmpeg
job done in one instance, with only cat for loopback.
Output from it goes to overlay, which supports only yuv and rgb. Not
sure, if that's reason.
But still - w
I managed to get it working with overlay, using tee & named fifo.
In first attempt I had two ffmpeg instance:
1 - creating new image, merging with accumulator image and sending to
tee, which makes split job, redirecting it to second ffmpeg.
2 - takes output from ffmpeg-1 and shifts, preparing for
