processFlip set lastFlipVblank to the vblank count after the host present
returned. When the acquire, the wait for an earlier presentation or the
present itself crossed the next vblank, the next flip's target moved out
by one vblank even when that flip was already queued: a frame ready in
17 ms was shown for 33 ms, and a title that advances its logic by a fixed
step per presented frame ran in slow motion.
A flip takes effect at the vblank it is released at, so that vblank is
now the one the next flip's interval counts from, recorded when the flip
is released. Flip events, flipStatus and flipPendingNum are unchanged.
Test: video_out_flip_tests pacing (needs a GPU and a display) submits
the first flip at flip rate 2, whose presentation opens the device and
spans several vblanks, and checks that lastFlipVblank is 3.
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