Resurrect vertex formats dropped for macOS < 10.13 #4549

In #231 we dropped a bunch of vertex formats that required macOS 10.13+.

In #1069 (comment), a year ago, we decided to require 10.13.

I knew we were missing some random stuff that we should have added back when we decided that, but that's a pretty major one. (It was not even 2 years a lot of years in between the two.)

We should add these back. List but with names updated:

MTLVertexFormatUChar --> uint8
MTLVertexFormatChar --> sint8
MTLVertexFormatUCharNormalized --> unorm8
MTLVertexFormatCharNormalized --> snorm8

MTLVertexFormatUShort --> uint16
MTLVertexFormatShort --> sint16
MTLVertexFormatUShortNormalized --> unorm16
MTLVertexFormatShortNormalized --> snorm16
MTLVertexFormatHalf --> float16

MTLVertexFormatUChar4Normalized_bgra --> unorm8x4-bgra / unorm8bgra (spelling?)

All of these are required in Vulkan 1.0 and D3D FL11_0.

(The 3-component formats we removed for D3D12 still can't be added back.)