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.