Add layer observer based on raw-window-metal

[madsmtm]

After #6107, resizing is still broken, and the right way to fix it is to create a sublayer that resizes itself based on the superlayer using key-value observing (see this for an introduction).

I have written a crate for this already, called raw-window-metal and filed #6210, but that crate depends on objc2, which is still under review by Firefox/Mozilla. So in the meantime, let's fix this directly in Wgpu.

Fixes #7011 and a few other issues, and should be fairly cleanly backport-able to v23 or v24 if desired.

I didn't write all that much documentation and safety comments here, and was kinda sloppy in some places, because the hope is that this fix will be temporary.

Testing
Tested by cherry-picking onto v23.1.0, and running Bevy's mobile example in the iOS simulator.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy.
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.