Revert "Switch to KeithP's xorg/xserver tree for extra glamor"

This reverts commit 4c7ec11c3b071b1b4c75c6b68598b0a6c962063e.

KeithP's fork of xorg/xserver is no longer needed, as all current
commits are happening in git://anongit.freedesktop.org/xorg/xserver.
This ebuild is rapidly becoming superfluous since it is now essentially
identical to the x11 overlay's ebuild.  Will probably remove.