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