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.
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
layout: Don't double-count padding, etc, and don't draw
backgrounds/borders/etc. twice for inline-blocks.

Improves Reddit, GitHub, and many other sites.
layout: Refactor a few things, mostly just to obey the line limit.