Improve validation in graph editor

This changelist improves the document validation step in the graph editor, importing the standard libraries to allow validation of node interfaces as well as document syntax.

source/MaterialXGraphEditor/Graph.cpp

@@ -84,7 +84,6 @@ Graph::Graph(const std::string& materialFilename,
     _geomFilter.push_back(".gltf");
 
     _graphDoc = loadDocument(materialFilename);
-    _graphDoc->importLibrary(_stdLib);
 
     _initial = true;
     createNodeUIList(_stdLib);
@@ -177,6 +176,7 @@ mx::DocumentPtr Graph::loadDocument(mx::FilePath filename)
         if (!filename.isEmpty())
         {
             mx::readFromXmlFile(doc, filename, _searchPath, &readOptions);
+            doc->importLibrary(_stdLib);
             std::string message;
             if (!doc->validate(&message))
             {
@@ -4057,7 +4057,6 @@ void Graph::drawGraph(ImVec2 mousePos)
         std::string graphName = fileName.getBaseName();
         _currGraphName.push_back(graphName.substr(0, graphName.length() - 5));
         _graphDoc = loadDocument(fileName);
-        _graphDoc->importLibrary(_stdLib);
 
         _initial = true;
         buildUiBaseGraph(_graphDoc);