* [Metal] Sprinkle [return: Release] on all 'new*' selectors. Fixes#5941.
Also add tests for all the API I could figure out how to use.
Fixes https://github.com/xamarin/xamarin-macios/issues/5941.
* [tests] MTLFunctionConstantValues didn't have a default ctor until Xcode 9.
* [tests] Use a higher offset when calling MTLBuffer.CreateTexture to try to comply with the requirements for the API.
Hopefully fixes this assertion:
> 07:42:06.7701360 validateStrideTextureParameters:1512: failed assertion `Linear texture: bytesPerRow (64) must be aligned to 256 bytes'
which doesn't happen on my machine.
* Fix whitespace.
* Simplify nested usings.
* Fix availability correctly.
Rolf Bjarne Kvinge