Cherry-picking an EXA patch for 1.7-nominations

[Cyril Brulebois]

Hi,

Michel kindly explained the issue at hand in the Debian bug#576816
(http://bugs.debian.org/576816), and I tried cherry-picking
342f3689d17256c92cbfee079d24501d27aa1153 accordingly, on top of
1.7-branch. The conflicts are:
++<<<<<<< HEAD
++=======
+       /* During a fallback we must finish access, but we don't know the index. */
+       if (pExaScr->fallback_counter)
+           exaFinishAccess(&pPixmap->drawable, -1);
+ 
+       if (pExaScr->deferred_mixed_pixmap == pPixmap)
+           pExaScr->deferred_mixed_pixmap = NULL;
+ 
++>>>>>>> 342f368... exa: handle pixmap create/destroy in lower layers

Building fails if the latter is kept. The crash still occurs if the
former isn't kept (and that seems like something we want to have, and
kind of matches Michel's description of the issue). Tests were kindly
performed by Arthur Marsh.

Mraw,
KiBi.