Improve acid2. Fix line height calculation. Text fragments get correct enclosing element style.

* Enabled acid2 on mac + linux. Updated the reference image. The only difference from the
real acid2 now is the paint order and a 1 pixel horizontal offset on the nose.
* Change line-height to be calculated correctly.
* Apply enclosing element style to text fragments.
Don't render backgrounds for text fragments.
Made some DOM fields private.

Relevant to #2242.
layout: Support any `display` property in generated content, and allow
tables to clear floats.

Improves the GitHub header.
Improve the correctness of Root lifetimes

Use the new lifetime bounds feature of Rust to enforce that the
RootCollection field of Root outlives the jsref field. This still
doesn't enforce that the Root itself outlives the jsref field, and
doing this would require some sort of init dance.
Convert &JSRef -> JSRef in DisabledStateHelpers
Merge pull request #3489 from ProgramFOX/issue2242-2

Made some DOM fields private.

Reviewed-by: jdm
Merge pull request #3493 from zwarich/root-lifetimes

Improve the correctness of Root lifetimes

Reviewed-by: jdm
Merge pull request #3496 from zwarich/disabled-state-helpers-jsref

Convert &JSRef -> JSRef in DisabledStateHelpers

Reviewed-by: jdm
Remove unneeded clones and add some comments.
Merge pull request #3486 from glennw/inline-fixes

Improve acid2. Fix line height calculation. Text fragments get correct enclosing element style.

Reviewed-by: pcwalton
Merge pull request #3492 from pcwalton/clear-generated-content-table

layout: Support any `display` property in generated content, and allow

Reviewed-by: glennw
Fix some mutable variable warnings.
Merge pull request #3500 from glennw/warning-fix

Fix some mutable variable warnings.

Reviewed-by: pcwalton