Fixed #4127: Added a fast path to border-radius.

This will avoid drawing Arcs if there's no border-radius property set.
code review
auto merge of #4187 : cgaebel/servo/issue-4127, r=SimonSapin

This will avoid drawing Arcs if there's no border-radius property set.

Fixes #4127
Merge branch 'master' of github.com:servo/servo