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.
Update to latest dependency libraries for Vulkan SDK 1.1.92.
Generate Hologram demo dispatch files in fetchDependencies and remove them from git.
Update MoltenVK version to 1.0.27.
Update What's New document.
via vkGetPipelineCacheData(), vkCreatePipelineCache() & vkMergePipelineCaches().
Add the cereal serialization framework as a dependency.
Update documentation. Update project settings to Xcode 9.3.
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.
