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.
try to reset flows which need reflow, since reflow isn't yet idempotent
layout: Rewrite intrinsic inline-size and automatic table layout to
match L. David Baron's work-in-progress specification.

    http://dbaron.org/css/intrinsic/

Column spans are not yet supported.

This effectively adds support for percentage widths, and it also fixes
many bugs, improving the layout of Google and Wikipedia.
Implement media queries parser and matching. Improves mobile first sites like bootstrap3.
auto merge of #3609 : pcwalton/servo/tables, r=SimonSapin

http://dbaron.org/css/intrinsic/

Column spans are not yet supported.

This effectively adds support for percentage widths, and it also fixes
many bugs, improving the layout of Google and Wikipedia.

r? @SimonSapin
auto merge of #3640 : cgaebel/servo/incremental-flow-construction, r=pcwalton

This also hides the not-yet-working parts of incremental reflow behind a runtime
flag. As I get the failing reftests passing, I'll send pull requests for them one
by one.
Removes duplicate CSS selector matching logic.

Now that DOM/Flow traversals have been refactored out, the `recalc_style_for_subtree`
function in `css/matching.rs` can be removed, in lieu of just running the standard
`recalc_style_for_node` and `construct_flows` traversals sequentially. Now we
no longer have the maintenance headache of duplicating selector matching logic
in two places! \o/

r? @pcwalton
auto merge of #3610 : glennw/servo/media-queries, r=SimonSapin
auto merge of #3668 : cgaebel/servo/sequential-reflow, r=pcwalton

Now that DOM/Flow traversals have been refactored out, the `recalc_style_for_subtree`
function in `css/matching.rs` can be removed, in lieu of just running the standard
`recalc_style_for_node` and `construct_flows` traversals sequentially. Now we
no longer have the maintenance headache of duplicating selector matching logic
in two places! \o/

This passes reftests with both default arguments, and with `-y 1`.

r? @pcwalton
auto merge of #3672 : mrobinson/servo/background-color, r=zwarich

Instead of relying on a scene-wide background color, all layers can now
have their own background color.
Update rust-http to pick up latest additions.