Support vertex attribute offsets higher than the strides in Metal

[kvark]

There is a lot of test in CTS (e.g. "dEQP-VK.texture.filter.**") that try to use an attribute offset higher than the stride. For example, an offset of 64 on a buffer with stride 8. It is technically valid in Vulkan, but Metal doesn't like it.

One possible solution would be to detect this at pipeline creation before linking the shader, then assign additional buffer inputs to be used in the shader by those attributes... adjusting the binding model accordingly.

[kvark]

Having this implemented, I faced two strange/interesting sub-issues:

1. The original vertex buffer binding may no longer be needed if we remap it and nothing needs a base offset of 0. Metal doesn't like unused vertex bindings, so I had to implement the remapping aggressively to work on the full range of bindings as opposed to just handling the exceptional ones.
2. Still getting a Metal validation error like "Attribute at index 1: offset(4) + 12 must be <= buffers[2].stride(12).". What's not clear to me is where does the first "12" come from...