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.
Load external library versions using script instead of Git submodules.
SPIRV-Cross, Vulkan-LoaderAndValidationLayers & VulkanSamples versions recorded in files.
glslang version determined by Vulkan-LoaderAndValidationLayers.
SPIRV-Tools & SPIRV-Headers versions loaded by glslang.
When MoltenVK is built as part of the LunarG SDK, use external libraries
sourced from parent Vulkan-LoaderAndValidationLayers.
Use Vulkan headers from Vulkan-LoaderAndValidationLayers.
No longer generate Vulkan specification.
Update documentation.