awful.client.shape: Finish the shape image when done with it

This function queries the shape of a client and then does something with
it. This commit makes sure the image is finished afterwards, which means
that most of its memory is released immediately instead of waiting for
the garbage collector to collect it.

Related-to: https://github.com/awesomeWM/awesome/issues/1958
Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/client/shape.lua

@@ -51,6 +51,8 @@ function shape.get_transformed(c, shape_name)
         cr:set_source_surface(shape_img, border + l, border + t)
         cr:rectangle(border + l, border + t, geom.width - l - r, geom.height - t - b)
         cr:fill()
+
+        shape_img:finish()
     end
 
     if _shape then