Support shareable surfaces

[kvark]

Is your feature request related to a problem? Please describe.
We need to be able to use the rendering result of wgpu externally. Like, in Gecko, WebRender needs to show the WebGPU-rendered frame.

Describe the solution you'd like
Some sort of a way to "import" a shareable surface, visible as wgpu texture. This requires some coordination with gfx-rs.

Describe alternatives you've considered
Alternatively, we could expose native handles of textures, allowing the client to schedule their own copy into whatever shareable surface they need.

Additional context

• https://bugzilla.mozilla.org/show_bug.cgi?id=1708022
• https://bugzilla.mozilla.org/show_bug.cgi?id=1708023
• https://bugzilla.mozilla.org/show_bug.cgi?id=1708025

[jimblandy]

We've already implemented the "alternative" approach described above, exposing the platform-level objects via as_hal methods, so Firefox doesn't need this bug solved any more. But others may want to design something more portable, so we'll leave this issue open.

[cwfitzgerald]

I think this has been basically covered by #4067