Implement MetalDevice about fna

[tgjones]

FYI, Veldrid now has Metal support, using a custom C# wrapper compatible with .NET Core, no Xamarin.Mac in sight:

https://github.com/mellinoe/veldrid/tree/master/src/Veldrid.MetalBindings

[flibitijibibo]

Good to know we can sidestep both Objective-C and Xamarin if needed.

Since this is on my mind, I'll point out that the Vulkan Portability Initiative may render this backend unnecessary if it ever actually ships. Our D3D11 support via ANGLE and existing Vulkan-to-Metal layers have made me MUCH less concerned about writing entire vendor-locked backends when the layers seem to do a pretty good job and are probably healthier for the ecosystem anyway. And hey, it'd give us D3D12 as well.

But again, it'd need to ship. Still waiting for that GitHub repo, Khronos...

[flibitijibibo]

This will no longer be pursued as MoltenVK has been open sourced and has already been made compatible with SDL 2.0.8:

https://www.khronos.org/news/press/vulkan-applications-enabled-on-apple-platforms

https://hg.libsdl.org/SDL/rev/d97ab6d12404

Please refer to the VulkanDevice issue if you were looking to replace OpenGL on macOS.