Improvements to color transform handling

- Add support for a wider set of use cases for color space bindings, including surface shader nodes with filename-type inputs.
- Add support for scoped color space bindings in shader translation.
- Add a test suite example that covers newly-supported use cases.

Supporting changes include:

- Merge the protected method ShaderGraph::createSurfaceShader into ShaderGraph::create, allowing surface shader nodes to be handled in the same fashion as other nodes.
- Update the interface of ShaderGraph::createNode, allowing greater flexibility in accessing parent nodes during shader generation.  We've omitted a deprecated wrapper for this API update, as it would not be straightforward to write one in a memory-safe fashion, but fortunately this method only appears to be used within the ShaderGraph class.

resources/Materials/TestSuite/pbrlib/surfaceshader/network_surfaceshader.mtlx

@@ -0,0 +1,22 @@
+<?xml version="1.0"?>
+<materialx version="1.38">
+  <nodedef name="ND_network_surface" node="network_surface">
+    <input name="base_color_filename" type="filename" value="" />
+    <output name="out" type="surfaceshader" />
+  </nodedef>  
+  <nodegraph name="NG_network_surface" nodedef="ND_network_surface">
+    <tiledimage name="base_color_image" type="color3">
+      <input name="file" type="filename" interfacename="base_color_filename" />
+    </tiledimage>
+    <standard_surface name="standard_surface" type="surfaceshader">
+      <input name="base_color" type="color3" nodename="base_color_image" />
+    </standard_surface>
+    <output name="out" type="surfaceshader" nodename="standard_surface" />
+  </nodegraph>
+  <network_surface name="N_surfaceshader" type="surfaceshader">
+    <input name="base_color_filename" type="filename" value="resources/Images/grid.png" colorspace="srgb_texture" />
+  </network_surface>  
+  <surfacematerial name="N_surfacematerial" type="material">
+    <input name="surfaceshader" type="surfaceshader" nodename="N_surfaceshader" />
+  </surfacematerial>
+</materialx>

source/MaterialXGenShader/GenContext.h

@@ -125,6 +125,24 @@ class MX_GENSHADER_API GenContext
         return _closureContexts.size() ? _closureContexts.back() : nullptr;
     }
 
+    /// Push a parent node onto the stack 
+    void pushParentNode(ConstNodePtr node)
+    {
+        _parentNodes.push_back(node);
+    }
+
+    /// Pop the current parent node from the stack.
+    void popParentNode()
+    {
+        _parentNodes.pop_back();
+    }
+
+    /// Return the current stack of parent nodes.
+    const vector<ConstNodePtr>& getParentNodes()
+    {
+        return _parentNodes;
+    }
+
     /// Add user data to the context to make it
     /// available during shader generator.
     void pushUserData(const string& name, GenUserDataPtr data)
@@ -216,6 +234,7 @@ class MX_GENSHADER_API GenContext
     std::unordered_map<const ShaderOutput*, string> _outputSuffix;
 
     vector<ClosureContext*> _closureContexts;
+    vector<ConstNodePtr> _parentNodes;
 
     ApplicationVariableHandler _applicationVariableHandler;
 };