* don't worry about deep copying, because immutability
* don't need `attrs` parameter which is usually empty
* don't worry about resetting `v` entity version
For fills and shadows, style changes can happen only when
a new version of the feature is produced. If we include a
version number in the key, we need only call TagClasses on
the enter selection.
We cannot apply the same optimization for strokes, since
stroke style can depend on the tags of parent relations
as well.
