Reduce number of font instances and shaper structures created.

The font cache previously kept weak references to the fonts,
however in layout these fonts are dropped before they are
referenced again, so no caching was being used.

For now, just hold a strong reference in the cache, which means
that fonts will never be thrown out (this will need to be fixed
in the future).
Fix comment and and reference to bug #3300
Add 'mach clean'.
Merge pull request #3298 from glennw/fix-font-cache

Reduce number of font instances and shaper structures created.
layout: Don't crash on floated generated content.

It doesn't construct the float correctly, but at least it doesn't crash
anymore.

Fixes Reddit.

Closes #3287.
Merge pull request #3293 from pcwalton/floated-generated-content-crash

layout: Don't crash on floated generated content.
Implement basic support for display: inline-block.

This still needs a lot of work, but it covers the basic
cases and improves wikipedia while passing all existing tests.

Tweak reftest to deal with linux/travis black background.
build: Update submodules
Merge pull request #3315 from saneyuki/mach

Add 'mach clean'.; r=Manishearth
Merge pull request #3286 from glennw/inline_block

Implement basic support for display: inline-block.
Add support for tables that are floated.
Address review comments
Add more arguments to mach test commands
Merge pull request #3312 from mbrubeck/test-name

Add more arguments to mach test commands. r=jack
Merge pull request #3305 from glennw/float-table

Add support for tables that are floated.
Merge pull request #3317 from pcwalton/update-submodules

build: Update submodules
Don't fail converting invalid UTF8 when fetching JS source. Fixes #3302.
Fix incorrect path to about:failure source.
Stop messing with the case of the attribute name in AttributeHandlers::get_attribute.

This fixes a bug where GetAttributeNS would incorrectly match lower-case
attributes when called with an upper-case argument.
Merge pull request #3323 from Ms2ger/get_attribute

Stop messing with the case of the attribute name in AttributeHandlers::get_attribute; r=Manishearth