Vblank synced repaints on Nvidia

Fredrik had a cool solution idea for bringing Nvidia repaint scheduling in line with Mesa like Vblank event synchonization.

Basically the Nvidia-specific block-till-vblank call would be moved in a separate thread such that we can listen for an synthetic Vblank event on unblock coming from this thread.

It didn't make it into the initial composition rework patch series but could be added now still.