Support more browsers

[konsumlamm]

Currently, only Chromium-based browsers are supported, since OffscreenCanvas is used to render the game. It would be nice to have an alternate renderer in order to make it possible to run Rask in non-Chromium browsers (e.g. Firefox).

[nat-rix]

Are we other browsers yet?

• ❌ ❓ firefox: See https://bugzilla.mozilla.org/show_bug.cgi?id=1390089 for the (?) tracking issue for firefox.

  [Supported] Roughly 2020H2.

• ✔️ chromium, edge, opera and Samsung Internet: supported, see https://developer.mozilla.org/en-US/docs/Web/API/OffscreenCanvas (not tested by me).
• ❌ Trash Browsers

[TrueDoctor]

Path towards offscreen canvas fallback implementation:
Add the required Content Security Policy (CSP) headers required to work under firefox: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer/Planned_changes
(this can for testing purposes be disabled by setting a flag in about:config)
Fetch the worker.js to make it accessible from the main thread
execute the script directly from the main thread in case of the offscreen canvas not working