feat(video): switch vp8 in ffmpeg to realtime

[dgozman]

This switches vp8 to "realtime" mode that works fast, adapting to
the speed of incoming frames, and produces the best quality in can
given realtime constraints.

In practice, this gives 2x larger video files but no noticible quality
difference. It also eliminates huge delays for encoding the video.

Drive-by: document our ffmpeg option choices and add some links
to documentation for future use.

[yury-s]

For the record, I ran this script with different setting on Ubuntu 20.04 and got the following numbers:

• Video resolution == page resolution

with patch:
1 core:
VIDEO SAVE: 36392.871ms
4 cores:
VIDEO SAVE: 8765.019ms

current behavior:
1 core:
VIDEO SAVE: 109553.257ms
4 cores:
VIDEO SAVE: 35889.124ms

• Video resolution = 800x600

with patch:
1 core:
VIDEO SAVE: 6845.509ms
4 cores:
VIDEO SAVE: 1206.802ms

current behavior:
1 core:
VIDEO SAVE: 16504.808ms
4 cores:
VIDEO SAVE: 3164.540ms