Readonlypath: stringA path on the machine doing the rendering, read at render time.
Readonlyurl: stringA remote address, fetched at render time.
Refused unless the renderer was built to fetch. The core is
fetch-free by default and answers a URL with an error rather than
reaching the network on an input's say-so; the CLI needs --features net for the same reason.
Readonlybytes: Uint8ArrayThe encoded image itself, when the caller already holds it.
Where an image's bytes come from.