Track source/mask pixmaps more explicitly for Composite fallback regions.
In particular, make sure pExaScr->src/maskPix are cleared when the
corresponding pictures aren't associated with drawables, i.e. solid or
gradient pictures. Without this, we would in some cases associate the
source/mask region with unrelated pixmaps from previous Composite
fallbacks, resulting in random corruption. (bnc#786153, fdo#47266)
OBS-URL: https://build.opensuse.org/package/show/X11:XOrg/xorg-x11-server?expand=0&rev=418
