WIP: HLSL: add Append/ConsumeBuffer support

Test/hlsl.structbuffer.append.frag

@@ -0,0 +1,11 @@
+AppendStructuredBuffer<float4>  sbuf_a;
+ConsumeStructuredBuffer<float4> sbuf_c;
+
+AppendStructuredBuffer<float4>  sbuf_unused;
+
+float4 main(uint pos : FOO) : SV_Target0
+{
+    sbuf_a.Append(float4(1,2,3,4));
+
+    return sbuf_c.Consume();
+}