DOCS/encoding: make "What does not work yet" a proper RST section
lua: add mp.unregister_event() function

Someone requested this... I think.
video: Include better heuristics for guessing primaries

This incorporates vertical resolution and common aspect ratios to
distinguish between likely 525-line and likely 625-line content.
Merge branch 'master' of https://github.com/mpv-player/mpv