awful.mouse.snap: Finish temporary image

This function creates a temporary image surface to set the shape of a
wibox. After this commit, the image is now finished after use. This
results in most of the image's memory to be freed immediately instead of
waiting for the garbage collected to collect it.

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

lib/awful/mouse/snap.lua

@@ -66,6 +66,7 @@ local function show_placeholder(geo)
     cr:stroke()
 
     placeholder_w.shape_bounding = img._native
+    img:finish()
 
     placeholder_w.visible = true
 end