Intent to prototype: OffscreenCanvas

[Andrew Osmond]

Summary: Initial implementation of OffscreenCanvas for WebGL and BitmapRenderer. This allows them to be used on a worker thread.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1736177

Related metabug: https://bugzilla.mozilla.org/show_bug.cgi?id=1390089

Standard:

https://html.spec.whatwg.org/multipage/canvas.html#the-offscreencanvas-interface

https://www.khronos.org/registry/webgl/specs/latest/1.0/

Platform coverage: All

Preference: gfx.offscreencanvas.enabled, currently disabled by default.

Other browsers: Chrome has shipped it since 69.

web-platform-tests:

https://searchfox.org/mozilla-central/source/testing/web-platform/tests/html/dom/idlharness.worker.js

https://searchfox.org/mozilla-central/source/testing/web-platform/meta/html/canvas/offscreen/manual/the-offscreen-canvas

One important caveat is that we don't currently have Canvas2D support, so if something uses that on the worker thread as a helper/companion, you are out of luck. I am currently prototyping this, as it is sometimes used by WPTs and the WebGL conformance suite to compare against the WebGL canvas.

With that said, there is a demo I found that should be working, at least on Linux, hopefully elsewhere too, as of the most recent nightly, that illustrates some benefit to this:

https://chrisprice.io/offscreen-canvas/?100000