Auto merge of #12945 - Manishearth:bgarray, r=SimonSapin

Support multiple backgrounds in both servo and stylo

(Commits do not build individually, but split up for review)

These patches make all of the background- properties accept multiple values, and add the layout code to display them.

Still needs some cleanup, and some testing, but it seems to work.

r? @SimonSapin

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12945)
<!-- Reviewable:end -->

components/style/lib.rs

@@ -56,6 +56,7 @@ extern crate log;
 #[allow(unused_extern_crates)]
 #[macro_use]
 extern crate matches;
+extern crate num_integer;
 extern crate num_traits;
 extern crate ordered_float;
 extern crate quickersort;