A save wrote the path relative to the document only for linked images and
placed documents in the layers. Links inside symbols, graph designs,
patterns and opacity masks kept only their absolute path, so after the
document and its files moved to another folder together, those files were
not found.
- The save sets the relative path of every link the document holds.
- On open, a link inside that art whose file was found away from its path
takes the new path, as a link in the layers does, so the next save
writes the right relative path.
- Document::update_links_where changes only the links it picks and copies
only the nodes on the way to them.
Docs: docs/mcp.md (Linked images), docs/target-app-parity.md.