st-theme-node: Improve borders with gradient backgrounds

st-theme-node: Improve borders with gradient backgrounds

For gradient backgrounds, borders were implemented by filling the
background shape with the border color first, and then scaling down
the path to draw the background.

The result is not correct[0], which is especially visible if the border
width is greater than the border radius - so instead of scaling down
the original path, use a separate path for the background.

The result is consistent with the borders we draw for non-gradient
backgrounds, and much closer to the correct standard behavior.

[0] http://www.w3.org/TR/css3-background/#the-border-radius

https://bugzilla.gnome.org/show_bug.cgi?id=607500