HLSL: add methods to track user structure in texture return type.

Some languages allow a restricted set of user structure types returned from texture sampling
operations.  Restrictions include the total vector size of all components may not exceed 4,
and the basic types of all members must be identical.

This adds underpinnings for that ability.  Because storing a whole TType or even a simple
TTypeList in the TSampler would be expensive, the structure definition is held in a
table outside the TType.  The TSampler contains a small bitfield index, currently 4 bits
to support up to 15 separate texture template structure types, but that can be adjusted
up or down.  Vector returns are handled as before.

There are abstraction methods accepting and returning a TType (such as may have been parsed
from a grammar).  The new methods will accept a texture template type and set the
sampler to the structure if possible, checking a range of error conditions such as whether
the total structure vector components exceed 4, or whether their basic types differe, or
whether the struct contains non-vector-or-scalar members.  Another query returns the
appropriate TType for the sampler.

High level summary of design:

In the TSampler, this holds an index into the texture structure return type table:

    unsigned int structReturnIndex : structReturnIndexBits;

These are the methods to set or get the return type from the TSampler.  They work for vector or structure returns, and potentially could be expanded to handle other things (small arrays?) if ever needed.

    bool setTextureReturnType(TSampler& sampler, const TType& retType, const TSourceLoc& loc);
    void getTextureReturnType(const TSampler& sampler, const TType& retType, const TSourceLoc& loc) const;

The ``convertReturn`` lambda in ``HlslParseContext::decomposeSampleMethods`` is greatly expanded to know how to copy a vec4 sample return to whatever the structure type should be.  This is a little awkward since it involves introducing a comma expression to return the proper aggregate value after a set of memberwise copies.

glslang/Include/Types.h

@@ -80,7 +80,19 @@ struct TSampler {   // misnomer now; includes images, textures without sampler,
     bool   combined : 1;  // true means texture is combined with a sampler, false means texture with no sampler
     bool    sampler : 1;  // true means a pure sampler, other fields should be clear()
     bool   external : 1;  // GL_OES_EGL_image_external
-    unsigned int vectorSize : 3;  // return vector size.  TODO: support arbitrary types.
+    unsigned int vectorSize : 3;  // vector return type size.
+
+    // Some languages support structures as sample results.  Storing the whole structure in the
+    // TSampler is too large, so there is an index to a separate table.
+    static const unsigned structReturnIndexBits = 4;                        // number of index bits to use.
+    static const unsigned structReturnSlots = (1<<structReturnIndexBits)-1; // number of valid values
+    static const unsigned noReturnStruct = structReturnSlots;               // value if no return struct type.
+
+    // Index into a language specific table of texture return structures.
+    unsigned int structReturnIndex : structReturnIndexBits;
+
+    // Encapsulate getting members' vector sizes packed into the vectorSize bitfield.
+    unsigned int getVectorSize() const { return vectorSize; }
 
     bool isImage()       const { return image && dim != EsdSubpass; }
     bool isSubpass()     const { return dim == EsdSubpass; }
@@ -90,6 +102,7 @@ struct TSampler {   // misnomer now; includes images, textures without sampler,
     bool isShadow()      const { return shadow; }
     bool isArrayed()     const { return arrayed; }
     bool isMultiSample() const { return ms; }
+    bool hasReturnStruct() const { return structReturnIndex != noReturnStruct; }
 
     void clear()
     {
@@ -102,6 +115,9 @@ struct TSampler {   // misnomer now; includes images, textures without sampler,
         combined = false;
         sampler = false;
         external = false;
+        structReturnIndex = noReturnStruct;
+
+        // by default, returns a single vec4;
         vectorSize = 4;
     }
 
@@ -160,16 +176,17 @@ struct TSampler {   // misnomer now; includes images, textures without sampler,
 
     bool operator==(const TSampler& right) const
     {
-        return type == right.type &&
-                dim == right.dim &&
-            arrayed == right.arrayed &&
-             shadow == right.shadow &&
-                 ms == right.ms &&
-              image == right.image &&
-           combined == right.combined &&
-            sampler == right.sampler &&
-           external == right.external &&
-         vectorSize == right.vectorSize;
+        return      type == right.type &&
+                     dim == right.dim &&
+                 arrayed == right.arrayed &&
+                  shadow == right.shadow &&
+                      ms == right.ms &&
+                   image == right.image &&
+                combined == right.combined &&
+                 sampler == right.sampler &&
+                external == right.external &&
+              vectorSize == right.vectorSize &&
+       structReturnIndex == right.structReturnIndex;            
     }
 
     bool operator!=(const TSampler& right) const