Right now we update QtQuick views after a composition has rendered so
that we'll have them ready for our next frame.

One of my machines (With X11 + nvidia) was quite stuttery, profiling
shows we spend a huge amount of time with the CPU blocked waiting for
the vblank from the previous main compositing to finish swapping before
we could start processing the QtQuick rendering. Time when the CPU is
blocked is wasted time.

This patch does mean kwin's compositing cycle lasts longer but a recent
patch changes the rendering policy when we start a fullscreen effect anyway so
hopefully that should still give us enough time to render that first frame in time.


(cherry picked from commit 63bf8112)