Change handling of gradients

Some changes to the way we handle CSS gradients:

 * draw without padding, thus interpreting gradients as part of the
   background rather than as content

 * clip to (rounded) border area

 * draw the border along the gradient instead of trying to align the
   gradient layer with the background/border layer

 * use the border_image actor instead of the background_image one

https://bugzilla.gnome.org/show_bug.cgi?id=606257