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.