Auto merge of #3032 - servo:border-profile, r=kvark
Auto merge of #3032 - servo:border-profile, r=kvark

renderer: Fix border profiling.

Was broken by #3011.

Also while at it, avoid setting and unsetting blend modes unnecessarily.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/webrender/3032)
<!-- Reviewable:end -->

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender