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.frag

@@ -0,0 +1,23 @@
+struct sb_t
+{
+    float3 color;
+    bool   test;
+    bool   test2;
+}; // stride = 20
+
+StructuredBuffer<sb_t>  sbuf : register(c10);
+StructuredBuffer<float> sbuf2;
+
+float4 main(uint pos : FOO) : SV_Target0
+{
+    sb_t mydata = sbuf.Load(pos);
+
+    uint size;
+    uint stride;
+    sbuf.GetDimensions(size, stride);
+
+    if (sbuf[pos].test)
+        return float4(sbuf[pos].color + sbuf2[pos], 0);
+    else
+        return mydata.color.x + size + stride;
+}