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

This will avoid drawing Arcs if there's no border-radius property set.
code review
Have 'mach doc' copy Rust documentation.
Replace `mach serve-docs` by `mach browse-doc`, fix #3913
layout: Implement `opacity` per CSS-COLOR § 3.2.

This adds the infrastructure necessary to support stacking contexts that
are not containing blocks for absolutely-positioned elements. Our
infrastructure did not support that before. This minor revamp actually
ended up simplifying the logic around display list building and
stacking-relative position computation for absolutely-positioned flows,
which was nice.
auto merge of #4036 : pcwalton/servo/opacity, r=SimonSapin

This adds the infrastructure necessary to support stacking contexts that
are not containing blocks for absolutely-positioned elements. Our
infrastructure did not support that before. This minor revamp actually
ended up simplifying the logic around display list building and
stacking-relative position computation for absolutely-positioned flows,
which was nice.

This will need this PR: https://github.com/servo/rust-azure/pull/112 I have not updated the Cargo.lock file yet because I want the merge commit.

r? @glennw
f? @SimonSapin
auto merge of #4142 : SimonSapin/servo/refactor-doc, r=Manishearth
Implement Window.sessionStorage: Storage Task, Storage Methods (excluding Storage event, QuotaExceededError)
auto merge of #4157 : nkdalmia/servo/master, r=jdm

Changes
* Implemented Storage Task
* Used Storage Task in methods of storage.rs
* Updated webstorage test expectations

Pending Changes:
* Handle Storage Event
* Throw QuotaExceededError in case of failure for method setItem
* localStorage as alias of sessionStorage
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