[wip] MacOS build support

[pvh]

Here are some patches for a MacOS build of RealityMerge. I know there's more to do (and possibly some of this is wrong) but I wanted to share it now that I've seen it running at least once.

[pvh]

I split up my commits into hopefully explanatory bits but in case you run into trouble, there are several separate changes I made:

1. The currently recommended version of the VulkanSDK linked by the Godot team wasn't compatible with 4.2.1, so I bumped the dependency. You may have path problems. Hopefully not!
2. I added trivial macos build target support to the cmakefile.txt.
3. The ar tool provided by BSD is incompatible with the one used on Linux (and I guess Windows). The llvm-ar one is compatible but isn't included by default on MacOS. The README (should) explain how to set cmake to use the right path but if your build fails because of arguments to ar somewhere, that's what's up and what to look into.
4. There's something on this Mac platform that didn't like the way pointers were being cast. I'm not sure what to do about that but I think my lie to the compiler shut it up for now so at least we can defer solving this problem. Of the various things I did, this is the worst.

It's possible any of these will be wrong but it's late here and I've already sunk the better part of an afternoon into this so I hope it works for you. Enjoy! (Or don't!)

[cklokmose]

Has the PR become obsolete with @nikitalita's PR merged?