Fix width and height args for _cairo_xcb_connection_copy_area()

Found via static analysis.

Signed-off-by: Maks Naumov <maksqwe1@ukr.net>
Reviewed-by:  Uli Schlachter <psychon@znc.in>

src/cairo-xcb-surface-core.c

@@ -573,7 +573,7 @@ _cairo_xcb_surface_core_copy_boxes (cairo_xcb_surface_t		*dst,
 						 src->x0 + x1,
 						 src->y0 + y1,
 						 x1, y1,
-						 x2 - x2, y2 - x2);
+						 x2 - x1, y2 - y1);
 	    }
 	}
     }