DRI3/Present fixes for XServers 1.16 and 1.17rc - reviewed

[Mario Kleiner]

Ok, the final patchset. I made the changes Eric Anholt proposed,
retested, and got them reviewed by him. With those on top of
XServer 1.16.2, DRI3/Present works for me on all drivers and
backends (intel sna, uxa and nouveau exa, glamor), single and
dual-display fullscreen and windowed, with/without compositor,
vsynced or non-vsynced etc.

The patches apply cleanly to master, 1.17-rc and 1.16.2.

Thanks,
-mario