This is a rewrite for how style interfaces with its consumers in order to allow
different representations for an element snapshot.
This also changes the requirements of an element snapshot, requiring them to
only implement MatchAttr, instead of MatchAttrGeneric. This is important for
stylo since implementing MatchAttrGeneric is way more difficult for us given the
atom limitations. This also allows for more performant implementations in the
Gecko side of things.
As a follow-up, we could move all the data living under a mutex in the
SharedLayoutContext only in order to create the local context to the same place.
This should increase animation performance when there are multiple animations in
one page that happen to be on different threads.
These can't be supported in Servo as of right now, because I'm not
totally sure the accesses that should be done in layout would be
thread-safe.
It can be revisited later though.
This commit refactors the style crate to be completely independent of
the actual implementation and pseudo-elements supported.
This also adds a gecko backend which introduces parsing for the
anonymous box pseudo-elements[1], although there's still no way of
querying them.
https://mxr.mozilla.org/mozilla-central/source/layout/style/nsCSSAnonBoxList.h
