This support was always there in Metal. It was just hidden from public
view--but not from prying eyes on the Objective-C metadata.
I had initially thought they were using OpenGL/Vulkan values for the
logic operation. When that didn't work, I then tried D3D11 values. I
guess D3D11 makes sense, because the Intel GPU drivers were the first to
support this in Metal.
Move the definition of `MVK_USE_METAL_PRIVATE_API` into
`"MVKCommonEnvironment.h"` so we can use it from non-C++ code.
8d1303a57a