MVKConfiguration access is now global, and the VkInstance provided in the
vkGet/Set/MoltenVKConfigurationMVK() functions is ignored. This allows these
functions to be provided with a VkInstance object that originates from a
different Vulkan layer than MoltenVK, without risking breaking the API.
MVKConfiguration extended to cover all MoltenVK environment variables.
Move all environment variable declarations to MVKEnvironment.h.
Add MVKEnvironment.cpp to define config functions.
Cleanup .m files to use MVKCommonEnvironment.h instead of MVKEnvironment.h.
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