- MVKRenderSubpass add separate getDepthFormat() & getStencilFormat(),
and isDepthAttachmentUsed() & isStencilAttachmentUsed() and use
instead of testing pixel format for depth and stencil components.
- Add MVKRenderingAttachmentIterator class to consistently iterate,
and take actions, on the attachments in VkRenderingInfo to create
synthetic MVKRenderPass and extract image views and clear colors.
- Remove mvkCreateRenderPass() and mvkCreateFramebuffer() in favor
of additional constructors, and remove mvkGetDepthStencilFormat() in
favor of retrieving formats for separate depth and stencil attachments.
- MVKRenderpass constructors reorganize order of adding attachments and
subpasses, and connecting the two.
- Renmame MVKRenderPassAttachment to MVKAttachmentDescription.
- MVKPipeline reorganize member variables to minimize gaps in content
and remove unnecessary _isRasterizingDepthStencil member var (unrelated).
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