commit-timing-v1: Add new protocol

Another attempt at a protocol for adding presentation time information at commit time.

Previous work exists at !45 and !103 (closed) and perhaps elsewhere, but progress on those has been stalled for months to years.

I'm hoping we can move forward with a much smaller protocol (no feedback here, I'm hoping that can be added to the existing presentation feedback protocol, perhaps with the work in !199)

I'm also hoping things like "slop" or "duration" which exist in other proposals could be added to this protocol eventually, but really want to start this as just a simple "trigger this commit no sooner than" request - which I think is useful on its own.

(in that regard, is adding "vsync hints" a step too far?)