css: Don't got to the selector tree for change

css: Don't got to the selector tree for change

The tree is optimized for mimizing the decisions, and is built ahead-of-time.
That prevents us from taking advantage of the information in the matcher when
collecting changes.

So, instead do what we used to do for verification: Use the selector tree
for finding the superset matches, then just walk the rulesets to collect
the changes.

Since we are now recomputing the change masks much less frequently, this
slightly less optimized way of computing them is not a problem, and will
let us compute better results in the future, by improving the superset
matcher to be more precise.