Enable vsync traversals by default.

Improved how the various callbacks are managed and sequenced
to reduce code duplication.

Added a heuristic to avoid postponing traversals until
the next vsync frame if we did not actually do any drawing during
the previous frame.  This helps in the very common case where
drawing occurs in response to input.

Change-Id: I277d9eeaf50408f8745a3cfd181db1d140770658