Fix jerky animations for the XRender backend

The code controlling timing for the animations
assumes that all painting is finished when the
Scene has finished painting. Since the X protocol
is asyncronous we need to call XSync (and not XFlush)
to make sure all painting has finished.
Tested for more than 2 weeks without issues.

NB: This is the same commit as
d0cf7ff6da4c6f8bcc684dd6378e2af09d36bb06
due to style changes in kwin code I could not cherry-pick
it cleanly