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.
Remove some dead code.
Rename make_url/current_url to parse_url/base_url, to align with spec terminology.
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
layout: Refactor flow construction to move around large structs less.

5% perf win, and a net loss in lines of code.
script: Pass the correct parent node to pre_insert in insertBefore

Closes #1572.
Add a test file for insertBefore
Give up and set the initial size to be the same as the default size for reftests
Use synchronous image loading for ref test baseline writing
Update to rust-png with fix for truncation
tests: Add missing finish() and a title in insertBefore content test
auto merge of #1621 : abustany/servo/fix-insert-before, r=Ms2ger

Closes issue #1572
auto merge of #1623 : larsbergstrom/servo/inline_border_reftest, r=metajack

r? @metajack

These are all commits you've previously reviewed, but without the image_border change madness. We're seeing some intermittent reftest failures, so I'd like to separately land these.