Handle negative index in vertex processing

[kainino0x]

Also refactor the prose slightly so that this change makes more sense in context.

Fixes #5064

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[kainino0x]

Oh, Brandon is out. Jim, could you review?

[mwyrzykowski]

Doesn't the definition of vertexElementRelativeOffset still allow for negative vertexElementIndex? E.g., consider

|vertexElementIndex| * |vertexBufferLayout|.{{GPUVertexBufferLayout/arrayStride}} +
                |attributeDesc|.{{GPUVertexAttribute/offset}}.

as long as |attributeDesc|.{{GPUVertexAttribute/offset}} >= -|vertexElementIndex| * |vertexBufferLayout|.{{GPUVertexBufferLayout/arrayStride}} then as written I believe vertexElementIndex < 0 would still be allowed.

Is negative vertexElementIndex supported across all APIs? I could only find reference in the GL specification which says it is not. Specifically I would wonder which values are populated in vertex_index on the shader side.

[Kangz]

Is this PR still current?

[kainino0x]

We never fixed this so yes. But I'll have to revisit at some point to figure out what we were doing.