Mac: Complete refactoring of the Mac code using a handful of macros. This allows using the old mutex system for thread safety pre-mojave. On Mojave and later instead we create two functions, an entrypoint and an actual code function which will get called on the main thread. This avoids Apples new artifical checks that the code is running on the main thread, since it actually is running on the main thread even though the actual caller might not be.