The import gives an element an id when it needs to find it again in
usvg's tree (links, symbol uses, events, data attributes, and now
circles and ellipses). A made-up id could repeat one the file already
had, such as vectorcraft-id-0, so a <use> or paint reference to that
element showed the other one. Made-up ids now skip every id in the file.
Circles and ellipses in a file whose style sheet has an [id selector get
no made-up id and keep usvg's curves, so the selector matches the same
elements as before.