Enable usage of react-pdf inside a web worker

[carlobeltrame]

Thanks a lot for this shim! As I already mentioned in diegomura/react-pdf#1317, it has helped me to run react-pdf using vite, and even inside a Vue application.
In the meantime, we have started using web workers for rendering the PDFs for performance reasons. The approach from diegomura/react-pdf#464 has served as a basis there. However, I have had to add another shim, since in a web worker environment, window is not defined, but instead the global scope is called self.
This PR adds the fix I found to this library, in the safest way I could find. It should only have any effect if the code is currently running inside a web worker. We are able to transparently switch our implementation between running on the main thread and running in the worker, and both variants work fine with this small addition.

[thekevinbrown]

Looks great, thanks for the patch!

[thekevinbrown]

This is now published in v1.0.5. Let me know if you have any trouble!

[aclec]

Hello, I used your method to render my pdf in a web worker.
However I cannot use "require" like you because an error is raised and by using "await import", I have the impression that my component is not imported because I have no error but my console.log afterwards the import never runs.

[carlobeltrame]

@aclec I have also had problems similar to what you describe, with Firefox not supporting dynamic imports of web worker scripts correctly, while in Chromium it worked fine. For now, I solved them by using vite-plugin-worker and using the worker-iife mode of that plugin: https://github.com/ecamp/ecamp3/blob/devel/frontend/src/components/print/print-react/generatePdf.js#L3
The iife part will generate an "immediately invoked function expression", which leads to the same behaviour as if I were using an async import, but with the added benefit that it works in Firefox.
The relevant but trivial vite config of that plugin: https://github.com/ecamp/ecamp3/blob/devel/frontend/vite.config.js#L17..L18

Additionally, I added import 'raf/polyfill' to get rid of some requestAnimationFrame-related warnings from React, which were popping up when using React in the worker.

Unfortunately, raf/polyfill also requires either global or window, so the fix I made in this PR here ended up not helping me. In other words: vite-plugin-shim-react-pdf modifies the code in react-pdf. Before importing react-pdf, I need to import React. Before React, I need to import raf/polyfill. And raf/polyfill already requires the global or window variable to be defined (I don't remember which one). So as a consequence, I still have to define window and global manually in my application: In this file, line 4 manually imports the extension I made in this PR, line 5 imports raf/polyfill and line 6 imports React and react-pdf.

I am also using Comlink to simplify the boilerplate code for communication between main thread and worker, but that should be optional.
Futher I have had some issues with fonts in the rendered PDF, so I had to make sure that Vite includes all required fonts in the production bundle, by importing all fonts in the main thread script.
Also, I had to be very careful not to import anything unexpected in the worker. E.g. in my case, Vue can never be imported in the worker, because Vue cannot run inside a worker as far as I am aware. As a consequence, my worker also cannot import any other packages that rely on Vue (Vuex, vue-i18n, ...).

This current setup works for me both during development and in a productive build, in Firefox, Chrome, Chromium, Edge and even Firefox mobile. Let me know if you need more explanations or if something is unclear.