zink: tweak xfb slot mapping in ntv

ARB_enhanced_layouts allows specifying overlapping variable locations for xfb outputs, so we need to explode the arrays here to a full 128 components so we can do per-component mapping

sometimes this fails though, as in the case where xfb is just selecting a single component from a vec but still considering the whole slot, and so for those cases we just decrement our array index until we get to the base, which will be within 3 components