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.
When the display goes to sleep IDXGIOutput::WaitForVBlank returns S_OK
immediately causing the VBlankThread to consume a core entirely with the
VBlank related messaging.
To limit this problem, we use the same technique as Chromium presently
does, and we sleep for 1 ms, if the time between the last two VBlank
events was less than a ms. This limits the VBlankThread messaging rate
of about 50.000/s on an Intel 13600 to 1000/s.
