Auto merge of #1989 - jrmuizel:yaml-filters, r=glennw
Auto merge of #1989 - jrmuizel:yaml-filters, r=glennw

Write out stacking context filters

Not writing out opacity explains why our yaml wrench numbers
sometimes looked better than our binary recording numbers.

<!-- 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/1989)
<!-- Reviewable:end -->

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