mi: Simplify a conditional in miHandleExposures

miHandleExposures does two things: computes the region for which to
generate expose events, and (if the destination is a window) paints the
exposed regions with the background. The bit of this conditional we're
deleting here asserts that the source is either a pixmap or a window
without backing store. The only other possibility is a window _with_
backing store. In the old backing store implementation, this was where
you would recover bits from backing store. Since our "backing store" is
the redirected window pixmap, we know we've already copied all we could,
because CopyArea had already seen the entire window pixmap. So now in
that third case, we are still drawing to a pixmap (so there's no
background to paint) and we are still not generating events, so we can
exit early.

The comment above the function about recovering bits from backing store
is clearly misleading, so delete that too.

mi/miexpose.c

@@ -112,10 +112,6 @@ the region package can call this.
 non-existent areas to non-obscured areas of the destination.  Paint the
 background for the region, if the destination is a window.
 
-NOTE:
-     this should generally be called, even if graphicsExposures is false,
-because this is where bits get recovered from backing store.
-
 */
 
 RegionPtr
@@ -140,10 +136,7 @@ miHandleExposures(DrawablePtr pSrcDrawable, DrawablePtr pDstDrawable,
     Bool extents;
 
     /* avoid work if we can */
-    if (!pGC->graphicsExposures &&
-        (pDstDrawable->type == DRAWABLE_PIXMAP) &&
-        ((pSrcDrawable->type == DRAWABLE_PIXMAP) ||
-         (((WindowPtr) pSrcDrawable)->backStorage == 0)))
+    if (!pGC->graphicsExposures && pDstDrawable->type == DRAWABLE_PIXMAP)
         return NULL;
 
     srcBox.x1 = srcx;