This restore a feature that was available in Awesome 2.1-3.2.
The reason margin is implemented rather than use a container is to
be able to make the background smaller than the bar.
The current progressbar code dates from a time when Awesome had
a very limited drawing API. This commit first re-write the
algorithm to remove the workaround used to draw the border using
full rectangles only. It then add support for outer and inner
shapes with their respective border settings.
This commit also add clip support. This is enabled by default, but
could be disabled to have the bar taller than the background.
Emmanuel Lepage Vallee