Auto merge of #3487 - gw3583:less-caching, r=kvark
Auto merge of #3487 - gw3583:less-caching, r=kvark

 Only cache tiles when they have had the same content > 2 frames.

This prevents tile caching doing redundant blits if the content
is changing every frame. This fixes a lot of the performance
regression on pages / benchmarks that have content which is
animated / changing every frame.

It still allows caching of tiles within those pages that are
static, giving the best of both worlds.

<!-- 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/3487)
<!-- Reviewable:end -->

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