Fixes the table_percentage_width_a.html reftest with incremental reflow turned on.
Dump the flow tree with println, instead of error
auto merge of #3715 : cgaebel/servo/fix-table-percentage-width, r=pcwalton

This also enables incremental reflow by default. \o/

r? @pcwalton
Update rust-cssparser to revision required by cssom branch.
auto merge of #3745 : glennw/servo/update-cssparser, r=jdm

r? @jdm
Update rust-core-text, handle empty font collections. Fixes #3703.
layout: Rewrite text and inline fragment handling during flow
construction to avoid cloning and moving flows so much.

Besides amounting to a 5%-10% win on a page with a lot of text, this
simplifies and refactors the text layout code.
layout: Don't destroy the flow tree when resizing the window
gfx: Make fonts that fail to match hit the cache too.

50%-100% layout perf improvement on CNN.
gfx: Don't needlessly push and pop clip rects all the time.

15% painting improvement on CNN.
auto merge of #3750 : glennw/servo/fix-3703, r=pcwalton

r? @pcwalton @metajack
Enable Opera's script scheduling tests.
auto merge of #3754 : Ms2ger/servo/enable-script-tests, r=Manishearth
auto merge of #3722 : pcwalton/servo/flow-construction-overhaul, r=glennw

This is a grab bag of performance improvements that significantly improve style recalculation, layout, and painting on a few static pages.

Let me know if you'd like me to split this PR up.

r? @glennw
more efficient preorder DOM traversals
auto merge of #3728 : cgaebel/servo/dump-flow-tree-with-println, r=mrobinson

r? @mrobinson
auto merge of #3744 : cgaebel/servo/efficient-preorder-traversal, r=pcwalton

This also tackles some nearby FIXMEs. `traverse_preorder` is used in a LOT of
DOM functions..

r? @pcwalton
Usage of JSRef<Attr> in before_remove_attr & after_set_attr

JSRef<Attr> does not require allocating a DOMString for value, which are
unused in most cases. It also provides more access to Attr data.