Fix the creation of DRI2 buffers for pixmaps

Testing with gnome-shell revealed a problem. We need to migrate
pixmaps into UMP buffers in order to allow the GLESv2 based
compositing manager to actually access the content of redirected
windows, rendered by X11 applications into offscreen pixmaps.

Just to make sure that we don't add any unneeded overhead for 2D
(neither extra CPU cycles nor the increase for unrelated pixmaps
memory footprint), a hash table (currently uthash [1]) is used
for connecting DRI2-enabled pixmaps with UMP buffers. The lookups
are only performed on DRI2 buffer creation and pixmap destruction.

1. http://troydhanson.github.com/uthash/

Reported-by: Michal Suchanek <hramrach@gmail.com>
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>