Add VK_KHR_separate_depth_stencil_layouts and enable its features.
Metal generally ignores image layouts, so nothing further needed.
Update Whats_New.md and MoltenVK_Runtime_UserGuide.md documents
with recently-added extensions.
Passes almost all supported separate_layouts CTS tests, with the
few remaining tests failing due to issues with unrelated capabilities.
Unrelated cleanup:
- MVKDevice enable extensions before features.
- MVKDevice reorder ivar declarations to reduce
memory layout gaps, and define default values.
- Rename VkSemaphoreStyle to MVKSemaphoreStyle to
remove potential conflicts with Vulkan Vk name space.
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