Add documentation about the scrolling model
Make sure reftests aren't passing because nothing == nothing

In the test harness, assert that the two images aren't all white.

Fixes #3481
Do not compound iframe margins when positioning

Instead of taking margin size into account twice when positioning
layers, just rely on the absolute position calculated during display
list construction.
Add support for local font faces. Improves fonts on rust lang and guide.
Don't borrow CharacterData.data from layout.

This should fix the most frequent intermittent wpt failure.
auto merge of #3676 : Ms2ger/servo/no-text-borrow, r=Manishearth

This should fix the most frequent intermittent wpt failure.
auto merge of #3673 : mrobinson/servo/iframe-margin, r=pcwalton

Instead of taking margin size into account twice when positioning
layers, just rely on the absolute position calculated during display
list construction.
Create an almost-correct form_owner() for convenience
Add form submission via input element
Add click handler for <input type=submit>
Filter out buttons which aren't the submitter while constructing the dataset
Update wpt expectations
Address review comments
auto merge of #3652 : Manishearth/servo/form-button, r=jdm

Partially fixes #3647
Allow individual layers to render their own background color

Instead of relying on a scene-wide background color, all layers can now
have their own background color.
script: Use atom comparison in more places, especially for attributes.

75% improvement in style recalc for Guardians of the Galaxy.
auto merge of #3675 : glennw/servo/local-fonts, r=pcwalton
try to reset flows which need reflow, since reflow isn't yet idempotent
auto merge of #3623 : pcwalton/servo/use-atoms-2, r=jdm

75% improvement in style recalc for Guardians of the Galaxy.
auto merge of #3634 : mrobinson/servo/scrolling-doc, r=jdm