Clamp-to-border support on Metal

@kvark commented:

See https://developer.apple.com/documentation/metal/mtlsamplerbordercolor?language=objc
there are 2 limitations:
1. the mode is only available on macOS 10.12+
2. it only supports 3 predefined colors

@cdavis5e commented:

> 2. it only supports 3 predefined colors

The same colors that base Vulkan itself supports, and the same ones supported by D3D12 for static samplers. Unlike Vulkan, Metal makes no distinction between floating-point border colors and integer border colors.

No repository files, tests, or entry points are named. Start with the linked Apple Metal documentation and determine the project's intended clamp-to-border support, including the macOS 10.12 limitation and three predefined colors; done requires an agreed implementation scope and validation criteria.

Written by the indexing model from the issue text.