Screen('WaitBlanking'): Small fix for PsychOSGetVBLTimeAndCount() fallback.

This path didn't update tvbl and time of last vbl, so returned wrong
'framesSincelastWait' return value and baseline.

Not crucial, as this function is seldomly used, and the specific
fallback path normally not executed in the first place.

-> No rebuild of mex files. Fold it into next regular Screen() rebuild.