expose the underlying metal device

[ccbrown]

This facilitates the ability to lock and utilize the underlying Metal device directly.

My need is very similar to the one described in https://github.com/gfx-rs/gfx/issues/3698, in which it was mentioned that functionality like this could be added to the individual backends and that PRs were appreciated.

PR checklist:

• [x] make succeeds (on *nix)
• [x] make reftests succeeds
• [x] tested examples with the following backends:

[bors[bot]]

Build succeeded:

• Android Stable
• Dependency Check
• iOS Stable
• MacOS Nightly
• MacOS Stable
• Ubuntu Nightly
• Ubuntu Stable
• Web Assembly
• Windows Nightly
• Windows Stable