awful.tooltip: Finish temporary image

For setting the shape of the tooltip, this code creates an image surface
describing the wanted shape. After this commit, this image surface is
finished when it is no longer needed. This results in most of the
image's memory to be freed immediately instead of only later when the
garbage collector collects the image surface.

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

lib/awful/tooltip.lua

@@ -137,6 +137,7 @@ local function apply_shape(self)
     s(cr, w, h, unpack(self._private.shape_args or {}))
     cr:fill()
     wb.shape_bounding = img._native
+    img:finish()
 
     -- The wibox background uses ARGB32 border so tooltip anti-aliasing works
     -- when an external compositor is used. This will look better than