Add reporting of damage by DRI drivers when the extension support is available.

Add reporting of damage by DRI drivers when the extension support is available.

With this, tools like ximagesrc in gstreamer correctly see updates from GL
rendering.  Support requires that the Xdamage library be current (but will be
disabled if not present) plus a new X Server with support for the new
XDamagePost request.  libGL now has a new interface version, and also links
against libXdamage and libXfixes to support it, but backwards compatibility
is retained.

Currently, all drivers report damage at SwapBuffers time through common code --
front buffer rendering doesn't result in damage being reported.  Also, the
damage is against the root window, as our drivers don't yet render to backing
store when they should (composited environments).