Allow no core when custom backend enabled

[sagudev]

Connections
Fixes #7329

Description
Currently wgpu assumes on native only we always need core, but that's not the case now that we have custom backend. Users of custom backend might not want to bring core into deps tree.

Testing
There is a new test and there are also existing tests.

Squash or Rebase? Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[sagudev]

I think we will need some platform-deps like we use in wgpu-core. cc @cwfitzgerald

[sagudev]

I think we will need some platform-deps like we use in wgpu-core. cc @cwfitzgerald

At least if we want that selecting backends will also select core on native, although we might want to let users specify core feature that actually brings in wgpu-core.

[cwfitzgerald]

Yeah this is why I didn't really try to solve it - the conditional dependency made things much more complicated, as you need all features to pass through but only conditionally, it's a real mess.

[sagudev]

Yeah this is why I didn't really try to solve it - the conditional dependency made things much more complicated, as you need all features to pass through but only conditionally, it's a real mess.

Yeah, I give up on this.