Fix surfaces only compatible with first enabled backend

[Wumpf]

Connections

• Fixes DX12 adapter does not support surface when multiple backends are specified in the instance #5324

Description
Both instances and surfaces are supposed to be created for all backends. This has some major drawbacks (see e.g. #2384 - creation of one halsurface may preclude creation of another halsurface), but this is how things were assumed to be working. Only that they didn't and we only created a single AnySurface with only the first successfully create hal surface!

This PR sets things right again and removing AnySurface in the process since there's not much use to it anymore.
We now create hal surface for each available hal instance. Error handling had to change a bit for this (not super happy with it, but will do).

Some small renames & doc additions as well.

Testing

• Checked the repro case in DX12 adapter does not support surface when multiple backends are specified in the instance #5324
• confirmed to fix [wgpu-core] request_adapter fails when requesting with a specific backend #5289

Checklist

• Run cargo fmt.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[cwfitzgerald]

LGTM