Update QtQuick views at the start of the frame

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.