Auto merge of #913 - kvark:cache, r=glennw,emilio
Auto merge of #913 - kvark:cache, r=glennw,emilio

Dummy sCache texture input

Fixes #907

r? @glennw

<!-- 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/913)
<!-- Reviewable:end -->

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