Make converter output seekable WEBM

[sgfn]

what this does:

• reencode the video
• cap used threads at 8
• set variable bitrate around 1.5M
• put MKV seek cues at the front of the file
• set GOP size to 150 -- @ 25 fps: generate keyframe every 6 secs

[sgfn]

dunno if we should merge to main or somewhere else, I needed to bump minors in ex_webrtc and live_ex_webrtc to test it out on glitch main