Fix up node creation and node instance interface input value assignment.

Author: kwokcb

source/MaterialXGenShader/ShaderGraph.cpp

@@ -576,6 +576,14 @@ ShaderGraphPtr ShaderGraph::create(const ShaderGraph* parent, const string& name
             if (nodeInput)
             {
                 ValuePtr value = nodeInput->getResolvedValue();
+                if (!value)
+                {
+                    InputPtr interfaceInput = nodeInput->getInterfaceInput();
+                    if (interfaceInput)
+                    {
+                        value= interfaceInput->getResolvedValue();
+                    }
+                }
                 if (value)
                 {
                     const string& valueString = value->getValueString();

source/MaterialXGenShader/ShaderNode.cpp

@@ -349,7 +349,7 @@ void ShaderNode::initialize(const Node& node, const NodeDef& nodeDef, GenContext
                 InputPtr interfaceInput = nodeInput->getInterfaceInput();
                 if (interfaceInput)
                 {
-                    portValue = interfaceInput->getValue();
+                    portValue = interfaceInput->getResolvedValue();
                 }
             }
             const string& valueString = portValue ? portValue->getValueString() : EMPTY_STRING;