Each layer a multi-layer mask covers gets a copy of the mask's path, so
a fill kept on that path painted in every layer, over the art the mask
covers in the layers before. The mask's fill is read as a path of its
own again, where the mask's path is, and the copies have no fill. A
fill read apart from a compound clipping path is a compound path whose
subpaths do not clip and have ids of their own. A fill read apart, its
subpaths and their points count toward the importer's object limit.
Docs: docs/file-format-parity.md.