Nit, this isn't a polyfill

This doesn't work as expected on mac/safari as discussed in this other issue.

[> [Mac/safari] will work, but use normal canvas without offscreen and web worker](https://github.com/ai/offscreen-canvas/issues/31#issuecomment-1013018932)

So then this isn't a polyfill then? This is a library that wraps offscreen canvas if available.

To me a polyfill implements the API for platforms that don't support it using the standard interface. This library requires users to a non-standard interface.

Furthermore it would be ideal if the polyfill could actually provide the desired functionality, in this case being able to offload canvas work to another web worker.

I think this could be done with iframes and proxies.

```mermaid
sequenceDiagram
main->>iframe: create canvas in iframe
main->>worker: use this offscreen canvas proxy
worker->>iframe: ctx.draw...
worker->>iframe: ctx.draw...
main->>+worker: getImageData
worker->>+iframe: forwardImageData
iframe-->>-main: ImageData
```