HLSL: implement 4 (of 6) structuredbuffer types

This is a partial implemention of structurebuffers supporting:

* structured buffer types of:
*   StructuredBuffer
*   RWStructuredBuffer
*   ByteAddressBuffer
*   RWByteAddressBuffer

* Atomic operations on RWByteAddressBuffer

* Load/Load[234], Store/Store[234], GetDimensions methods (where allowed by type)

* globallycoherent flag

But NOT yet supporting:

* AppendStructuredBuffer / ConsumeStructuredBuffer types
* IncrementCounter/DecrementCounter methods

Please note: the stride returned by GetDimensions is as calculated by glslang for std430,
and may not match other environments in all cases.

Test/hlsl.structbuffer.byte.frag

@@ -0,0 +1,13 @@
+
+ByteAddressBuffer sbuf;
+
+float4 main(uint pos : FOO) : SV_Target0
+{
+    uint size;
+    sbuf.GetDimensions(size);
+
+    return sbuf.Load(pos) +
+        float4(sbuf.Load2(pos+4), 0, 0) +
+        float4(sbuf.Load3(pos+8), 0) +
+        sbuf.Load4(pos+12);
+}