st-theme-node: Support non-uniform border-radii

st-theme-node: Support non-uniform border-radii

Non-uniform border-radii are already supported when using a gradient
background, this patch adds support for solid colors as well.

The currently applied technique of using corner textures and filling
the remaining area with rectangles is extended, so that each corner is
padded with rectangles to the size of the largest corner.

Add border-radius.js to test cases, to test non-uniform border-radii
with both solid color and gradient backgrounds.

https://bugzilla.gnome.org/show_bug.cgi?id=631091