Move shaders into shader library target

[colincornaby]

This PR does several things:

• It moves the shaders into a proper Metal shader library target when using the Xcode generator.
• It compiles the shaders twice - once for Metal 2.1 (macOS 10.14) and once for Metal 2.3 (macOS 11.0.) The Metal 2.3 build of the shaders can take advantage of special features when running on Apple Silicon.
• plMetalDevice now dynamically loads the correct compiled shader archive based on the host version of macOS
• The shader library is now cached instead of being re-created each time a shader is needed. (This change is cleanup thats long overdue.)

I'd like some feedback on the implementation in the CMake - and feedback on this change requiring CMake 3.28.

[colincornaby]

You probably need to assert in the metal pipeline that the CMake version is at the minimum required for this feature to work.

What would be the best way to do that? A different cmake_minimum_required or some sort of different check?

[Hoikas]

What would be the best way to do that? A different cmake_minimum_required or some sort of different check?

We may not want to use cmake_ minimum_required() because this has side effects related to setting policies, and we may not want to have different policy scopes playing with our mind. It may be better to test and error if the version is too low:

if(${CMAKE_VERSION} VERSION_LESS 3.9)
    message(FATAL_ERROR "boo")
endif()

[colincornaby]

Just checking in on this PR. I made some changes a few weeks back but haven't seen any feedback yet.

[Hoikas]

Looks like this PR needs to be rebased to build.

[colincornaby]

I went ahead and rebased pulling in @dpogue's latest Mac fixes. Waiting to see if the build is green.

[colincornaby]

It looks like builds are passing again on this PR.