- Fix runtime failure on Metal versions that don't support dynamic attribute stride.
- Add MVKCommandEncoder::encodeVertexAttributeBuffer() consolidation function.
- Remove unnecessary validations that will be caught by Vulkan validation layers.
- To reduce memory, remove command class and pools for rendering commands that
are not supported, and perform no validation.
- Document extension conformance limitations in MoltenVK_Runtime_UserGuide.md.