This seeks to address #1175, where a containing widget with a transparent
background, but with a border, won't show the border. It seems that at the
heart of the Compositor it's taking `is_transparent` (of the widget) as one
of the indicators that mean that it won't be visible. This wouldn't normally
be the case, in that a widget could be transparent (in the background) but
could have visible content.
Note that some snapshot tests failed with this change, but no material
difference was found in those failing snapshot tests.
af1168b4ee