730: Move backend selection to `Instance::new()` r=kvark a=Kimundi

**Connections**
Addresses gfx-rs/wgpu-rs#337
Corresponding wgpu-rs PR: gfx-rs/wgpu-rs#385

**Description**
This makes early backend selection possible in `Instance::new`, by only attempting to instantiate requested backends. The actual selection in `pick_adapter()` (necessarily) remains unchanged, and can be used to further restrict the request.

**Testing**
Unsure what to do here
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Non-trivial functional changes would need to be tested through:
  - [wgpu-rs](https://github.com/gfx-rs/wgpu-rs) - test the examples.
  - [wgpu-native](https://github.com/gfx-rs/wgpu-native/) - check the generated C header for sanity.

Ideally, a PR needs to link to the draft PRs in these projects with relevant modifications.
See #666 for an example.
If you can add a unit/integration test here in `wgpu`, that would be best.
-->


Co-authored-by: Marvin Löbel <loebel.marvin@gmail.com>