Add support for texElementImage2D

[cabanier]

HTML-in-Canvas is a new feature that allows pages to render HTML content to pixels in Canvas 2D, WebGL and WebGPU. The changes to the API are defined here.
This feature has been requested by countless developers and it's very exciting to finally have a solution that is easy to use and privacy/security preserving.

HTML-in-Canvas is in active development with support from the major browser vendors.
@foolip is adding the Canvas 2D APIs and the privacy/security mechanisms to the HTML specification and I'd like to add the new WebGL (and WebGPU) APIs to their respective specs.

@kenrussell , should we have a discussion first on how to proceed?

[kenrussell]

@cabanier yes, can you make tomorrow's WebGL working group meeting and we'll try to discuss this in real time with (hopefully) all of the browser vendors?

[kenrussell]

Note #3752 contains the first prototype IDL.

The linked issue in the WICG's html-in-canvas repository had a question about reducing the number of parameters to the function. I think they should remain as-is, matching the signatures of other WebGL entry points which upload objects from JavaScript and the DOM (images, video frames, etc.) because (1) the controls over things like internal format may be needed by applications and (2) reducing the number of parameters would cause an odd divergence between this new API and the preexisting ones.