Auto merge of #871 - servo:kvark-image, r=glennw
Auto merge of #871 - servo:kvark-image, r=glennw

Fix the image tex-coord clamping

The invalid argument order we use now would still clip the lower bound but ignore the higher bound.

r? @glennw
cc @jrmuizel

<!-- 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/871)
<!-- Reviewable:end -->

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