The intent is to use this extension on vc4 to allow X11 to do overlapping
CopyArea() within a pixmap without first blitting the pixmap to a
temporary.  With associated glamor patches, improves x11perf
-copywinwin100 performance on a Raspberry Pi 3 from ~4700/sec to
~5130/sec, and is an even larger boost to uncomposited window movement
performance (most copywinwin100 copies don't overlap).

v2: Fix glIsEnabled() on the new enums.
v3: Drop the local spec since I'm upstreaming the spec.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>