Layers were a feature of the legacy drawing path. If we re-add them at
some point, it probably makes more sense to make them a product of
display list inspection.
This change also remove a bunch of dead painting code.
A couple of changes here:
* Remove the option to unset with the dirty bit setters.
* Add an explicit API for setting text node style.
* Hoist has_changed handling into the restyle damage setter and text node style setter.
* Make set_style take a non-Option.
It's not possible to implement a Ref::map equivalent method on AtomicRefCell
while having AtomicRefCell implemented on top of RwArc. We could potentially
reimplement AtomicRefCell to be more like RefCell and add a Ref::map equivalent
method, but I (and pcwalton) think we should try just cloning a few extra
Arcs at these callsites instead.
MozReview-Commit-ID: 6H8vAWguO3z
