[metal] Use raw-window-metal to do layer creation

[madsmtm]

EDIT: raw-window-handle's implementation, as described below, was used in #7026. This PR now only updates to use the actual raw-window-handle crate.

You know, all that code and documentation I just wrote in #6107? Let's throw it away into a library, so that wgpu, ash, vulkano, i-slint-renderer-skia and so on can all benefit from it.

raw-window-metal is that library (existed previously, I've updated it in rust-windowing/raw-window-metal#19).

This makes Wgpu depend transitively on objc2, so the concerns about doing that apply here as well, see also #5641.

Description
Use the raw-window-metal crate to do layer creation. This uses much the same approach as we do currently, except that it also uses observers internally, which fixes resizing on high DPI screens when using Wgpu together with an NSView using auto-layout.

Though it probably won't matter much, it's also more reliable to update the scale factor this way, rather than haphazardly in configure.

The logic in raw-window-metal could be re-implemented in Wgpu to avoid the dependency, however I deemed it complex enough that I thought it wiser to centralize the implementation in one place.

Testing
See #6107 (comment).

Checklist

• Run cargo fmt.
• Run cargo clippy.
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.
  • I did not deem this to be necessary, since the bulk of this has already landed in Add layer observer based on raw-window-metal #7026.

[Wumpf]

very appreciated, having that code as part of raw-window-metal sounds great to me :)

As pointed out this will have to wait for the objc2 pr to be approved. cc: @jimblandy
(therefore not putting ✔️ just yet so this doesn't get merged ahead of time)

[polina4096]

I've tested this and it still adds some noticeable (especially in comparison) with the way it was before previous PR (#6107) had been merged. I've gone into a bit further details here #6107 (comment).

[jimblandy]

This depends on #5641. In our meeting today, the wgpu maintainers decided to mark #5641 as draft, since we'll be getting back to that in 2025. So, marking this as draft as well.