To better support pipeline layout compatibility between pipelines with differing
quantities of descriptor sets, move the buffer indexes used by implicit buffers to
the top end of the Metal buffer index range, below vertex and tessellation buffers.
MVKPipeline calculates implicit buffer indexes based on vertex and tessellation
buffers required by pipeline, instead of based on descriptors in MVKPipelineLayout.
MVKPipeline track buffer index counts consumed by MVKPipelineLayout, to validate
room for implicit buffers.
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