Previously, the vblank's thread loop would block on each iteration until
the current async callback had finished, at which point a new async
callback would be immediately triggered.
The new implementation only waits on the vblank event. If a vblank
callback is still in progress the next time the vblank event is
signalled, we assume the last frame is overrunning and avoid sending a
new async update.
The intention of this change is to avoid saturating the message thread
with expensive vblank callbacks. It's also nice to remove a lock,
although that's just an incidental change.