Allow building and running without Metal on MacOS

[nategraf]

Metal is currently required as a build dependency on MacOS even if it will not be used. As a result,
the codebase cannot be built without it.

This PR refactors the use of Metal to gate building dependant code behind the "metal" feature flag.
As an alternative to the current method, this means users may have to explicitly enable "metal" as a
feature at build-time (if the turn off defeault features).

[flaub]

I think the challenge here is how to convince cargo to build on non-macos platforms in such a way that doesn't include the metal crate. The previous attempt was using the target.'cfg(target_os = "macos")'.dependencies] approach because of this problem. It'd be interesting to see if we can feature flag this dependency.

[nategraf]

Superseded by #348