[Metal] Add a way to create a device and queue from raw resources in wgpu-hal

[AdrianEddy]

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
Related to #3145 and #2320

Description
This PR adds device_from_raw and queue_from_raw in addition to the existing texture_from_raw. This allows to get the MTLDevice and/or MTLCommandQueue from external application, and use them to process external memory/texture in wgpu.
My use case involves OpenFX plugin for DaVinci Resolve video editor, and Resolve provides MTLTexture and MTLCommandQueue pointers, which I then use in a plugin written in Rust and wgpu.

With these 2 new functions, one can create wgpu::Device and wgpu::Queue from a raw pointer to MTLCommandQueue the following way:

use foreign_types::ForeignType;
use wgpu_hal::api::Metal;

let mtl_command_queue: *mut metal::MTLCommandQueue = ...;

let (device, queue) = unsafe {
    let mtl_queue = metal::CommandQueue::from_ptr(mtl_command_queue);
    let mtl_device = mtl_queue.device();

    adapter.create_device_from_hal(wgpu_hal::OpenDevice::<Metal> {
        device: <Metal as wgpu_hal::Api>::Device::device_from_raw(mtl_device.to_owned(), wgpu::Features::empty()),
        queue: <Metal as wgpu_hal::Api>::Queue::queue_from_raw(mtl_queue)
    }, &wgpu::DeviceDescriptor {
        label: None,
        features: wgpu::Features::empty(),
        limits: wgpu::Limits::default()
    }, None).unwrap()
};

Testing
I tested these changes with pointers provided by the DaVinci Resolve, and also with pointers provided from Qt and QML renderer, both cases work as intended.

[codecov-commenter]

Codecov Report

Merging #3338 (ac3bd87) into master (aa46e82) will increase coverage by 0.41%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3338      +/-   ##
==========================================
+ Coverage   64.10%   64.51%   +0.41%     
==========================================
  Files          86       86              
  Lines       42591    42597       +6     
==========================================
+ Hits        27301    27480     +179     
+ Misses      15290    15117     -173     

Impacted Files	Coverage Δ
wgpu-core/src/hub.rs	60.67% <0.00%> (-0.31%)	⬇️
wgpu-hal/src/dx12/mod.rs	27.27% <0.00%> (ø)
wgpu-hal/src/vulkan/mod.rs	56.37% <0.00%> (ø)
wgpu-core/src/device/mod.rs	66.73% <0.00%> (ø)
wgpu-hal/src/dx12/command.rs	73.46% <0.00%> (+0.02%)	⬆️
wgpu-hal/src/auxil/dxgi/conv.rs	76.02% <0.00%> (+0.33%)	⬆️
wgpu-hal/src/vulkan/adapter.rs	76.77% <0.00%> (+0.33%)	⬆️
wgpu/src/backend/direct.rs	56.87% <0.00%> (+0.38%)	⬆️
wgpu-hal/src/vulkan/device.rs	75.84% <0.00%> (+0.43%)	⬆️
wgpu/src/context.rs	53.32% <0.00%> (+0.58%)	⬆️
... and 11 more

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[cwfitzgerald]

Looks good enough for now - we need to go through and audit all of these external interface functions later anyway.

[cwfitzgerald]

Whoops, wrong button

[AdrianEddy]

hmm, something went wrong here, I think I messed up the branches, give me a moment

[AdrianEddy]

Fixed, sorry about that. This change should only add device_from_raw and queue_from_raw to the metal backend. Please squash when merging