frame clock: Track if paint is caused by thaw or not

frame clock: Track if paint is caused by thaw or not

When we get to a paint cycle we now know if this was caused by a
thaw, which typically means last frame was drawn, or some other event.

In the first case the time of the cycle is tied to the vblank in some
sense, and in the others it is essentially random. We can use this
information to compute better frame times. (Will be done in later
commits.)