Multiple display list support
layout: Use the concurrent hash map for the leaf sets.

64% improvement in style recalc.
Implement document.compatMode

Spec:
http://dom.spec.whatwg.org/#dom-document-compatmode

This is a sub-task for #1428.
auto merge of #1579 : ibnc/servo/multiple_display_lists, r=larsbergstrom

Fixes: #1538

Still needs to be rebased.
layout: Consolidate passes in the sequential path that were consolidated
in the parallel path.
auto merge of #1598 : pcwalton/servo/sequential-mirrors-parallel, r=larsbergstrom

in the parallel path.

r? @larsbergstrom
auto merge of #1600 : pcwalton/servo/use-concurrent-hash-map, r=larsbergstrom

64% improvement in style recalc.

r? @larsbergstrom
Failed to link -lpng while make reftest is fixed.
auto merge of #1599 : brunoabinader/servo/document-compatmode, r=Ms2ger

Spec:
http://dom.spec.whatwg.org/#dom-document-compatmode

This is a sub-task for #1428.
auto merge of #1603 : xiongmao86/servo/master, r=larsbergstrom

I failed at make check, and find the problem in check.mk
layout: Enforce that flow construction is thread-safe.

Closes #1584.
auto merge of #1605 : pcwalton/servo/harden-layout-more, r=metajack

Closes #1584.

r? @metajack
Remove duplicate imports from render_task.rs.
Add more documentation for layout.

+ Annotate functions with CSS Reference sections.
+ Clean up comments and whitespace.
Remove unnecessary ClipDisplayItems in box_.rs

For ScannedTextBox and ImageBox, the ClipDisplayItem's child_list is
currently always empty.

ClipDisplayItem is used to implement overflow hidden and should only be
created for block containers, as per
http://www.w3.org/TR/CSS2/visufx.html#propdef-overflow

Take care of the case when a BlockFlow has no children display items -
like an ImageBox with "display: block".
auto merge of #1578 : pradeep90/servo/stacking-context, r=larsbergstrom,larsbergstrom

For ScannedTextBox and ImageBox, the ClipDisplayItem's child_list is
currently always empty.

ClipDisplayItem is used to implement overflow hidden and should only be
created for block containers, as per
http://www.w3.org/TR/CSS2/visufx.html#propdef-overflow
auto merge of #1606 : Ms2ger/servo/unused-import, r=jdm
implement position:relative
auto merge of #1613 : ksh8281/servo/pos_rel, r=larsbergstrom

implement position:relative

this pr implement #792
it gives same result with firefox in "src/test/ref/position_relative_a.html"
r? @larsbergstrom