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.
auto merge of #4437 : Adenilson/servo/fixInsetOutsetBlack03, r=jdm

Freshly rebased.