Make HasDisplayHandle optional in WindowHandle

[MarijnS95]

Connections
Followup to #8012 (comment) and rust-windowing/softbuffer#298 (comment) (CC @cwfitzgerald @dhardy)

Description
While true for some window objects like winit's Window object, not everyone is or should be required to return their display handle in the same place as the window handle, by means of implementing both RawDisplayHandle and RawWindowHandle on the same object.

Additionally some APIs, specifically most/all GL(ES) context creation APIs require the RawDisplayHandle up-front to create a context that is compatible with that "compositor connection".

For this reason callers should no longer be required to pass a single object that implements both HasDisplayHandle and HasWindowHandle, allowing the former to become Optional in favour of taking the one stored (and kept alive!) in the Instance.

HasDisplayHandle can still be passed in this position however, because APIs like Vulkan have completely pushed out compositor integration via surfaces to the edge of the API and can technically let one instance or device render into multiple different RawDisplayHandle "compositor connections" concurrently.

Testing

$ WGPU_BACKEND=gl cargo r -p wgpu-example-02-hello-window
$ WGPU_BACKEND=gl cargo r -p wgpu-examples -- hello_triangle
$ WGPU_BACKEND=gl RUST_LOG=wgpu_hal=debug cargo r -p player # Changed code but don't have a file to play and test it yet

Squash or Rebase?

Doesn't matter.

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.