Oxipng multi-threaded mode fails when run from main thread.

[jamsinclair]

Describe the bug Oxipng multi threaded code doesn't work when not wrapped in a worker.

This happens due to a known issue with threading solutions with wasm and rust (https://github.com/GoogleChromeLabs/wasm-bindgen-rayon#caveats).

To Reproduce Steps to reproduce the behavior:

1. Copy the with-vite example
2. Modify the code to import and use oxipng in the main js script
3. Set in the development server the following headers:

   headers: {
   "Cross-Origin-Opener-Policy": "same-origin",
   "Cross-Origin-Embedder-Policy": "require-corp",
   }

Expected behavior

We should update our wrapper code to not run multi-threaded code unless in a dedicated worker context.