remove @ from ScannedTextBox.run
make font/platform/gfx/font_context safe
remove @ in LayoutTask.FontContext
fix some code
layout: Remove `DisplayBoxes` to save memory and as one step toward an
`@`-free layout
layout: Remove the ability to put boxes in display list items.
auto merge of #1362 : pcwalton/servo/rm-display-boxes, r=larsbergstrom

`@`-free layout

r? @larsbergstrom
layout: Refactor `try_append_to_line` to eliminate duplicate code.
remove FontContxtHandle.clone
auto merge of #1356 : ksh8281/servo/remove_@_in_LayoutTask.FontContext, r=pcwalton

Remove @ in LayoutTask.FontContext, ScannedTextBox.text_run
#1334
layout: Refactor `InlineFlow::assign_height` to pull out horizontal
position calculation into a separate function.
Revert "auto merge of #1356 : ksh8281/servo/remove_@_in_LayoutTask.FontContext, r=pcwalton"

This reverts commit e8ffac13d7e0ebc0701b87b774491b2b1b895607, reversing
changes made to db923feffe4ef1f15c34c2a50acdf74a4321e2d4.

Reverting this change because FreeType is *not* thread safe. See the
documentation here:

http://www.freetype.org/freetype2/docs/reference/ft2-base_interface.html

"In multi-threaded applications, make sure that the same FT_Library
object or any of its children doesn't get accessed in parallel."

We will need to use a `MutexArc` instead.
auto merge of #1365 : pcwalton/servo/minor-layout-refactoring, r=metajack

r? @metajack
removing @ from flowtree. (but cloning boxes when creating a display list)