Define MVK_MACCAT build macro and use it to conditionally compile code to align
with build features and capabilities of Mac Catalyst platform on macOS 11.0+.
Treat Mac Catalyst as minor variation of macOS 11.0.
Update documentation.
Currently only support Mac Catalyst on macOS 11.0+, to avoid complexities of
deselecting iOS features and capabilities for Mac Catalyst on previous macOS versions.
Mac Catalyst (and Simulators) require use of XCFrameworks.
Currently unable to generate a dylib for Mac Catalyst.
Create fat builds of static, dynamic & framework libraries if both iOS
and simulator versions have been created from separate manual Xcode builds.
Refactor scripts for creating fat libraries to reuse across projects.
Add MVK_BUILT_PROD_DIR to replace use of BUILT_PRODUCTS_DIR in most scripts
to allow flexibility across per-platform compilation.
Refactor build scripts.
Update paths to MoltenVKGLSLToSPIRVConverter framework in demos.
Fix rare build race condition on MoltenVKShaderConverter link to MoltenVK.
