Some pixmaps (window pixmaps and scratch pixmaps) don't have the
drawable->id set and thus DRI2 gets confused when using that field
for looking up the DRI2 drawable.  Go back to using privates for getting
at the DRI2 drawable from a DrawablePtr.  We need to keep the resource
tracking in place so we can remove the DRI2 drawable when the X resource
it was created for goes away.  Additionally, we also now track the DRI2
drawable using a client XID so we can reclaim the DRI2 drawable even if
the client goes before the drawable and doesn't destroy the DRI2 drawable.

Tested-by: Owen W. Taylor <otaylor@fishsoup.net>
Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>