Frame timing

This results in proper, end-to-end handling of tracking prediction, including frame time.

It involves the compositor, instead of session, keeping track of frame times. Then, instead of a fixed prediction interval, the real time difference between predicted frame time and the tracker timestamp is used (the fixed prediction interval now becomes a "static prediction" value that's added to this duration - essentially, "extra latency" in the tracker.)

Doesn't actually wait for times yet, but does work on both Vive Pro and HDK2, and is "more correct" than the previous behavior.