Fix UiEdge node order in Graph Editor (#2770)

This changelist fixes a swapped argument order in the `UiEdge` constructor, where the upstream and downstream nodes were inverted relative to the usage of the constructor in the codebase.

Callers of the `UiEdge` constructor have been using the following convention, as seen in the `Graph::createEdge` method:

```
UiEdge newEdge = UiEdge(upNode, downNode, connectingInput);
```

But the constructor itself has been expecting arguments in the reverse order, as seen here:

```
UiEdge(UiNodePtr uiDown, UiNodePtr uiUp, mx::InputPtr input)
```

This changelist updates the arguments of the `UiEdge` constructor and all dependent code, making the semantics of `UiEdge::getUp` and `UiEdge::getDown` consistent across the Graph Editor codebase.

Author: jstone-lucasfilm

source/MaterialXGraphEditor/UiNode.cpp

@@ -106,18 +106,18 @@ UiNodePtr UiNode::getConnectedNode(const std::string& name)
     {
         if (edge.getInputName() == name)
         {
-            return edge.getDown();
+            return edge.getUp();
         }
-        else if (edge.getDown()->getName() == name)
+        else if (edge.getUp()->getName() == name)
         {
-            return edge.getDown();
+            return edge.getUp();
         }
     }
     for (const UiEdge& edge : _edges)
     {
         if (edge.getInputName().empty())
         {
-            return edge.getDown();
+            return edge.getUp();
         }
     }
     return nullptr;

source/MaterialXGraphEditor/UiNode.h

@@ -23,24 +23,24 @@ using UiPinPtr = std::shared_ptr<UiPin>;
 class UiEdge
 {
   public:
-    UiEdge(UiNodePtr uiDown, UiNodePtr uiUp, mx::InputPtr input) :
-        _uiDown(uiDown),
+    UiEdge(UiNodePtr uiUp, UiNodePtr uiDown, mx::InputPtr input) :
         _uiUp(uiUp),
+        _uiDown(uiDown),
         _input(input)
     {
     }
     mx::InputPtr getInput() const
     {
         return _input;
     }
-    UiNodePtr getDown() const
-    {
-        return _uiDown;
-    }
     UiNodePtr getUp() const
     {
         return _uiUp;
     }
+    UiNodePtr getDown() const
+    {
+        return _uiDown;
+    }
     std::string getInputName() const
     {
         if (_input != nullptr)
@@ -54,8 +54,8 @@ class UiEdge
     }
 
   private:
-    UiNodePtr _uiDown;
     UiNodePtr _uiUp;
+    UiNodePtr _uiDown;
     mx::InputPtr _input;
 };