Auto merge of #337 - glennw:opt-glyph-hash, r=pcwalton
Auto merge of #337 - glennw:opt-glyph-hash, r=pcwalton

Switch glyph hashset to use Fnv.

<!-- 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/337)
<!-- Reviewable:end -->

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