Fix start and end points for linear gradients with angle specified.
Previously, this was most noticeable with 45deg gradients, where the gradient would end too early, and the remainder was filled with a solid color.
(This also fixes gradients on webrender, which relies on the start and stop points being correct).
<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/8094)
<!-- Reviewable:end -->
b8f196f858
