work on gfx metal backend

[skallweitNV]

This PR implements enough of the Metal backend to run basic compute shaders:

• support for enumerating metal adapters
• basic support for buffer, texture and sampler resources
• basic support for shader program and compute pipeline creation
• basic support for command buffer + resource/compute encoder
• basic support for shader objects + layouts
• partial implementation for fence, swapchain, queries, vertex layout

This is by no means complete or well tested. It's enough to get a simple metal unit test working though, but support for testing will come in a separate PR.

[skallweitNV]

@jkwak-work thanks for the review. I'll address your comments in an upcoming PR.