For the recent mixed pixmaps changes, I failed to realize (or hit in my
testing) a problem which can occur if the driver doesn't provide an
UploadToScreen hook or provides one which can fail: There can be a crash
in exaMemcpyBox() because exaCopyDirtyToFb() passes pExaPixmap->fb_ptr to
exaCopyDirty(), but that's normally NULL with driver allocated pixmaps.
The solution is to make exaCopyDirty*() no longer rely on pExaPixmap->fb_ptr
but use pPixmap->devPrivate.ptr after PrepareAccess instead.
Fixes http://bugs.freedesktop.org/show_bug.cgi?id=24167 .