draw: fixes for vertex shaders outputting layer or viewport index

draw: fixes for vertex shaders outputting layer or viewport index

Mostly add a couple cases so we don't just check gs for this.
There's only one gotcha, the built-in vp transform in the llvm vs can't
handle it (this would be fixable though non-trivial due to vp index being
non-constant for the SoA outputs, but we don't use it if there's a gs
neither - the whole clip/vp transform integration there is suboptimal).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>