Remove backend_bits from initialize_adapter_from_env

[fornwall]

Checklist

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

Connections
#3792, initialize_adapter_from_env_or_default ignores backend_bits if WGPU_ADAPTER_NAME is not set

Description
As described in #3792, the initialize_adapter_from_env_or_default function currently ignores backend_bits if WGPU_ADAPTER_NAME is not set.

Instead of going towards the path of making these utility functions more complex (or adding e.g. backend_bits to RequestAdapterOptions), let's perhaps remove backend_bits from these functions completely, in favour of using InstanceDescriptor::backends? So instead of

let instance = wgpu::Instance::default();
let backends = wgpu::util::backend_bits_from_env().unwrap_or_else(wgpu::Backends::all);
let adapter = wgpu::util::initialize_adapter_from_env_or_default(&instance, backends, None)

the recommended way would be:

let backends = wgpu::util::backend_bits_from_env().unwrap_or_else(wgpu::Backends::all);
let instance = wgpu::Instance::new(wgpu::InstanceDescriptor { backends, ..Default::default() });
let adapter = wgpu::util::initialize_adapter_from_env_or_default(&instance, None)

What do you think?

Testing
By existing tests, or a utility program such as listed above.

[Wumpf]

I really like this idea, makes it much simpler. Thanks!

[Wumpf]

it took me really long to understand that enumerate_adapter does in fact not list backends that it didn't know about at creation. Should really be explicit on on its doc string I suppose.