Wait to dump flow tree until flow is complete

The flow tree is currently dumped for debugging purposes early on in
the flow process, so many values are still zero. If we wait to dump it
until later, the output will more accurately reflect the real flow tree.
auto merge of #3719 : mrobinson/servo/flow-dump, r=pcwalton

The flow tree is currently dumped for debugging purposes early on in
the flow process, so many values are still zero. If we wait to dump it
until later, the output will more accurately reflect the real flow tree.
Edits dom/bindings/DESIGN.md

This corrects and improves the dom/bindings/DESIGN.md document and also
corrects a grammatical error in a tests/reftest.rs message.
auto merge of #3720 : gilles-leblanc/servo/fix-grammatical-errors, r=jdm

This corrects and improves the dom/bindings/DESIGN.md document and also
corrects a grammatical error in a tests/reftest.rs message.
Don't crash on invalid utf-8 in the HTML parser.

This was regressed by the html5ever landing.
auto merge of #3712 : Ms2ger/servo/parser-crash, r=Manishearth

This was regressed by the html5ever landing.
Remove render backend option as it doesn't work and confuses people.
auto merge of #3711 : glennw/servo/cleanup-opts, r=pcwalton

r? @pcwalton @metajack
Use opts as a global, to avoid cloning and passing the struct all over the code.
auto merge of #3729 : glennw/servo/opts-cleanup, r=mbrubeck
Fixes the table_percentage_width_a.html reftest with incremental reflow turned on.
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
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.