HLSL: Partially flatten hierarchies, instead of all or nothing.

Fixes #1092.  Allows arrays of opaques to keep arrayness, unless
needed by uniform array flattening.
Can handle assignments of mixed amounts of flattening.

Test/hlsl.partialFlattenMixed.vert

@@ -0,0 +1,16 @@
+Texture2D tex[2];
+
+struct Packed {
+    int a;
+    Texture2D     membTex[2];
+    int b;
+};
+
+float4 main(float4 pos : POSITION) : SV_POSITION
+{
+    Packed packed;
+
+    packed.membTex = tex;
+
+    return pos;
+}