fix another bug where screenshots could end-up all black

SF transactions were always handled on VSYNC which allowed
the screenshot to sneak-in between closing the transaction
and vsync (before it's latched), resulting in a screenshot
with the previous state.

we now always force transactions to happen immediately
before screenhots.

Bug: 7552304
Change-Id: I0afc86b7e8366173daff5b9988bbb4d2a0f43860