Fix depth stencil attachment downcast for WebGPU

[grovesNL]

Checklist

• Run cargo clippy.
• Run RUSTFLAGS=--cfg=web_sys_unstable_apis cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file.

Connections
Fixes #3663 (maybe?)

Description
Untested but I believe we should be using as_ref() here instead

Testing
Untested

[mockersf]

seems to fix it for me 👍 (as in, I have other issues so still nothing renders, but at least not that error anymore)

[maxammann]

works!

[cwfitzgerald]

Great!

[cwfitzgerald]

Why does this work though?

[grovesNL]

@cwfitzgerald this is the subtle downcasting issue I mentioned on my other PR.

In this case data is basically Box<dyn Any>, but Box itself implements Any. This means we can easily break it if you accidentally use &Box<dyn Any> instead of &dyn Any, so we need something like as_ref() here to produce the correct type.

We could probably build some of this into our own downcast_ref wrapper. Ideally I'd like stronger guarantees on Data itself somehow, so we can verify that Any is carrying the correct type for the backend. Maybe we can wait to see how this looks after we make the Arc changes though.