[Xcb] Replacing cargo-culted Xlib DRI2 protocol with XCB.

[Eric Anholt]

All over the place people have copied XF86DRI protocol stuff because people
never made an Xlib library for it.  This is happening again with DRI2.
I'd like to cut that off now by converting them to XCB, so I threw together
this patch series.  It's not really tested because XVMC is broken
in 2/3 apps using it it seems, and the other one (mythtv) is a mess
to configure.

So, could the XCB folks review it to see if what I've done is sane (in
particular the struct for the GetBuffersWithFormat req which doesn't have an
equivalent in the xlib code, and the use of the reply strings in the XVMC
code)?  And Intel folks, do we see any issue with depending on XCB for this?