This will be needed for two other Vulkan 1.2 extensions,
`VK_KHR_depth_stencil_resolve` and
`VK_KHR_separate_depth_stencil_layouts`.
Most of this is just changing MVKRenderPass to store everything
internally in `RenderPass2` format. I also added some basic handling for
a few things I left out from earlier changes, input attachment aspect
masks and dependency view offsets. The former won't become important
until Metal supports depth/stencil framebuffer fetch. The latter won't
be needed until we start using untracked resources, and therefore need
to insert explicit fences and/or barriers between subpasses. We don't
need either right now, but I've handled them regardless.
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