Draft: commit-timing-v1: new protocol implementation

EDIT: CURRENTLY REWRITING. DON'T REVIEW YET. WIP implementation of commit-timing-v1.

Currently it only has round-to-nearest behavior in the 'presentation' stage (since the protocol details are still being worked out).

This includes some work on the headless backend so that compositors relying on it can make use of this extension (e.g., gamescope).

CC: @bl4ckb0ne :)