Add some time stamping to tracking

This is the first step of frame timing. I have also added the ability to basically compute the angular velocity for OpenHMD devices if it's not reported for some reason, which gives better results.

The caveat with that is that OpenHMD appears to return the same values twice in a row (two pose queries per frame?), which results in a velocity of 0. The previous velocity is correct, but will only be "active" half the frame - this means it looks a little bit laggier than natively getting angular velocity, but it's better than nothing.