panelCorner: Only overlap panel if necessary

panelCorner: Only overlap panel if necessary

The panel corners overlap the panel in order to hide the underline
used for active buttons where it is supposed to arc downwards
following the roundness of the corner.
Unfortunately this prevents us from using a transparent panel background,
as the overlapped area ends up with the wrong transparency. Work around
this limitation by only overlapping the panel if there is a visible
border.

https://bugzilla.gnome.org/show_bug.cgi?id=684573