layout: Implement `clip` per CSS 2.1 § 11.1.2.

Only the recommended, comma-separated syntax is supported.
auto merge of #4394 : pcwalton/servo/clip, r=SimonSapin

Only the recommended, comma-separated syntax is supported.

r? @SimonSapin
Similar to groove/ridge case, inset/outset would scale the color to
decorate the border. Scale fails when the color is black (i.e. zero),
thus this patch will fix this issue by using a fixed color.
Adding inset ref test.
Adding outset ref test.