WIP: HLSL: handle clip/cull distance array semantic matching

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

Test/hlsl.clipdistance-1.vert

@@ -0,0 +1,8 @@
+void main(out float4 pos : SV_Position, 
+          out float clip : SV_ClipDistance,  // scalar float
+          out float cull : SV_CullDistance)  // scalar float
+{
+    pos = 1.0f.xxxx;
+    clip = 0.5f;
+    cull = 0.51f;
+}