fix scheduling the repaints

repaints caused by effects so far polluted the timing calculations
since they started the timer on the old vsync offset
This (together with undercut timing) lead to multiple frames in
the buffer queue, and ultimately to a blocking swap

For unsynced painting, it simply caused wrong timings - leading to
"well, kinda around 60Hz - could be 75 as just well".

REVIEW: 112368

that part is fixed in 4.11.2