Fix MDL code gen after removing ClosureSourceCodeNode (AcademySoftwareFoundation#2465)

In AcademySoftwareFoundation#2400 the ClosureSourceCodeNode has been removed and replaced by the regular SourceCodeNode.
For that to work, the SourceCodeNode needs to emit dependent function calls to generate code for the inputs.

Author: krohmerNV

source/MaterialXGenMdl/Nodes/SourceCodeNodeMdl.cpp

@@ -77,6 +77,13 @@ void SourceCodeNodeMdl::emitFunctionCall(const ShaderNode& node, GenContext& con
     {
         const ShaderGenerator& shadergen = context.getShaderGenerator();
         const MdlShaderGenerator& shadergenMdl = static_cast<const MdlShaderGenerator&>(shadergen);
+
+        if (nodeOutputIsClosure(node))
+        {
+            // Emit calls for any closure dependencies upstream from this node.
+            shadergen.emitDependentFunctionCalls(node, context, stage, ShaderNode::Classification::CLOSURE);
+        }
+
         if (_inlined)
         {
             const MdlSyntax& syntax = static_cast<const MdlSyntax&>(shadergenMdl.getSyntax());