Add support for multioutput connection edits (AcademySoftwareFoundati…

…on#1506)

Incorrect connections were being stored on the MaterialX document when making / breaking connections with upstream nodes which have more than one output (mulitoutput nodes).

## Fixes

- Make sure to remove any reference on an input to an upstream output attribute when breaking links.
- Make sure to add the connection to an upstream output if the upstream node is multioutput. Otherwise use the current logic to connect to the node. 
- If we always add an output reference on the downstream input, then there is a validation warning if the upstream node is not multioutput but you specify an output on the downstream input. This might be worthwhile to remove as it's not really an error.

source/MaterialXGraphEditor/Graph.cpp

@@ -2606,7 +2606,33 @@ void Graph::AddLink(ed::PinId inputPinId, ed::PinId outputPinId)
                             {
                                 if (uiUpNode->getNode())
                                 {
-                                    pin->_input->setConnectedNode(uiUpNode->getNode());
+                                    mx::NodePtr upstreamNode = _graphNodes[upNode]->getNode();
+                                    mx::NodeDefPtr upstreamNodeDef = upstreamNode->getNodeDef();
+                                    bool isMultiOutput = upstreamNodeDef ? upstreamNodeDef->getOutputs().size() > 1 : false;
+
+                                    // This is purely to avoid adding a reference to an update node only 1 output,
+                                    // as currently validation consides adding this an error. Otherwise
+                                    // it will add an "output" attribute all the time.
+                                    if (!isMultiOutput)
+                                    {
+                                        pin->_input->setConnectedNode(uiUpNode->getNode());
+                                    }
+                                    else
+                                    {
+                                        for (UiPinPtr outPin : _graphNodes[upNode]->outputPins)
+                                        {
+                                            // set pin connection to correct output
+                                            if (outPin->_pinId == inputPinId)
+                                            {
+                                                mx::OutputPtr outputs = uiUpNode->getNode()->getOutput(outPin->_name);
+                                                if (!outputs)
+                                                {
+                                                    outputs = uiUpNode->getNode()->addOutput(outPin->_name, pin->_input->getType());
+                                                }
+                                                pin->_input->setConnectedOutput(outputs);
+                                            }
+                                        }
+                                    }
                                 }
                                 else if (uiUpNode->getNodeGraph())
                                 {
@@ -2716,6 +2742,9 @@ void Graph::deleteLinkInfo(int startAttr, int endAttr)
                     setDefaults(_graphNodes[upNode]->getInput());
                 }
 
+                // Remove any output reference
+                pin->_input->removeAttribute(mx::PortElement::OUTPUT_ATTRIBUTE);
+
                 pin->setConnected(false);
                 // if a value exists update the input with it
                 if (val)