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.
Add KhronosGroup/Vulkan-Portability as external dependency repo.
Add ExternalRevisions/Vulkan-Portability_repo_revision.
Add vk_extx_portability_subset.h header file to mvk_vulkan.h.
MVKImageView allow constructor with no image or device.
Add mvkVkComponentMappingsMatch() & mvkVkComponentSwizzlesMatch() functions.
Cleanup some local var init warnings.
