While adding geometry stage support for clip/cull, it transpired that the
existing clip/cull support was not setting the type of the result of indexing
into the clup/cull variable. That's a defect independent of the geometry
support, so to simplify the geometry PR, this is addressed separately.
It doesn't appear to change the generated SPIR-V, but that's probably down to
something else tolerating a bad input.
In HLSL, there are three (TODO: ??) dimensions of clip and cull
distance values:
* The semantic's value N, ala SV_ClipDistanceN.
* The array demension, if the value is an array.
* The vector element, if the value is a vector or array of vectors.
In SPIR-V, clip and cull distance are arrays of scalar floats, always.
This PR currently ignores the semantic N axis, and handles the other
two axes by sequentially copying each vector element of each array member
into sequential floats in the output array.
Fixes: #946
