glamor: Fix ignoring the ALU during SetSpans().

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Markus Wick <markus@selfnet.de>

glamor/glamor_setspans.c

@@ -48,7 +48,11 @@ _glamor_set_spans(DrawablePtr drawable, GCPtr gc, char *src,
         goto fail;
     }
 
-    /* XXX Shall we set alu here? */
+    if (gc->alu != GXcopy) {
+        glamor_fallback("SetSpans with non-copy ALU.\n");
+        goto fail;
+    }
+
     if (!glamor_set_planemask(dest_pixmap, gc->planemask))
         goto fail;