Add WGPU_BACKEND_VALIDATION environment variable

[cwfitzgerald]

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

Part of #2316

Description

This adds an env var which turns on backend validation no matter if we're in debug or release. This is useful for both forcing it on in release and forcing it off in debug.

Testing
None

[jimblandy]

Is an environment variable really how people want to work around this?

Would it make more sense to check the environment variable from something analogous to initialize_adapter_from_env, so that the app gets to opt into sensitivity to the environment? Usually we haven't hacked this kind of thing directly into Instance::new. It almost seems like this should be some sort of blocklist in the back end, rather than a magic var users need to know about to make things work for them.

Wearing my Mozilla hat, Firefox doesn't object, but we do have a general preference for less environmental sensitivity, because it makes problems that much harder to reproduce.

For all the above reasons, something more targeted would be better, like this one, which applies only to a specific back end, to address a specific bug:

wgpu/wgpu-hal/src/dx11/instance.rs

Line 5 in 6058676

let enable_dx11 = match std::env::var("WGPU_UNSTABLE_DX11_BACKEND") {