Add an additional meaning to the "dirty" flag. Now, if !dirty && !area, the

pixmaps's contents are undefined, so we won't need to upload the
    undefined contents in MoveIn. Use the ExaCheck* for async ops as well,
    so that dirty is always tracked. While the performance impact for my ls
    -lR test was not significant (though the avoiding-upload path was being
    hit), it's likely to be important for the upcoming Get/PutImage
    acceleration from ajax.

exa/exa_priv.h

@@ -119,8 +119,12 @@ typedef struct {
     int		    devKind;
     DevUnion	    devPrivate;
 
-    /* True if the in-screen copy has been modified compared to the
-     * system-memory copy.
+    /* If area is NULL, then dirty == TRUE means that the pixmap has been
+     * modified, so the contents are defined.  Used to avoid uploads of
+     * undefined data.
+     * If area is non-NULL, then dirty == TRUE means that the in-framebuffer
+     * copy has been changed from the system-memory copy.  Used to avoid
+     * downloads of unmodified data.
      */
     Bool	    dirty;
     unsigned int    size;