Add UDIM example and fix handling in Graph Editor (#2113)

Hoping to address #2007 - and also have some data to test while working on improving UDIM support in MaterialX.

Once I modified the textures it became clear that loading this in to MaterialXGraphEditor didn't actually load the correct images in to the correct materials.  They would all end up using the fallback material, so this PR includes a small fix to address that.

Author: ld-kerley

resources/Geometry/cube.obj

@@ -0,0 +1,41 @@
+# This file uses centimeters as units for non-parametric coordinates.
+g
+v -0.500000 -0.500000 0.500000
+v 0.500000 -0.500000 0.500000
+v -0.500000 0.500000 0.500000
+v 0.500000 0.500000 0.500000
+v -0.500000 0.500000 -0.500000
+v 0.500000 0.500000 -0.500000
+v -0.500000 -0.500000 -0.500000
+v 0.500000 -0.500000 -0.500000
+vt 0.99 0.01
+vt 0.99 1.01
+vt 1.01 0.01
+vt 2.01 0.99
+vt 2.01 1.01
+vt 1.99 0.99
+vt 1.01 1.01
+vt 2.99 2.01
+vt 1.01 1.99
+vt 1.99 1.99
+vt 2.99 0.01
+vt 2.99 0.99
+vt 0.01 0.01
+vt 0.01 0.99
+vt 2.01 0.01
+vt 0.99 1.99
+vt 1.99 0.01
+vt 0.01 1.01
+vt 0.01 1.99
+vt 0.99 0.99
+vt 2.01 2.01
+vt 1.99 1.01
+vt 1.01 0.99
+vt 2.99 1.
+g
+f 1/18 2/2 4/16 3/19
+f 3/3 4/17 6/6 5/23
+f 5/5 6/24 8/8 7/21
+f 7/7 8/22 2/10 1/9
+f 2/15 8/11 6/12 4/4
+f 7/13 1/1 3/20 5/14

resources/Images/grid_udim/grid.1001.png

@@ -0,0 +1,15 @@
+<?xml version="1.0"?>
+<materialx version="1.38">
+  <image name="image_color" type="color3">
+    <input name="file" type="filename" value="resources/Images/grid_udim/grid.<UDIM>.png" colorspace="srgb_texture" />
+  </image>
+  <geominfo name="cube_geom" geom="/">
+    <geomprop name="udimset" type="stringarray" value="1001, 1002, 1003, 1011, 1012, 1013" />
+  </geominfo>
+  <standard_surface name="cube_shader" type="surfaceshader">
+    <input name="base_color" type="color3" nodename="image_color" />
+  </standard_surface>
+  <surfacematerial name="cube_material" type="material">
+    <input name="surfaceshader" type="surfaceshader" nodename="cube_shader" />
+  </surfacematerial>
+</materialx>

resources/Images/grid_udim/grid.1002.png

@@ -403,15 +403,6 @@ void RenderView::updateMaterials(mx::TypedElementPtr typedElem)
         // Check for any udim set.
         mx::ValuePtr udimSetValue = _document->getGeomPropValue(mx::UDIM_SET_PROPERTY);
 
-        // Skip material nodes without upstream shaders.
-        mx::NodePtr node = typedElem ? typedElem->asA<mx::Node>() : nullptr;
-        if (node &&
-            node->getCategory() == mx::SURFACE_MATERIAL_NODE_STRING &&
-            mx::getShaderNodes(node).empty())
-        {
-            typedElem = nullptr;
-        }
-
         // Create new materials.
         if (!typedElem)
         {
@@ -422,6 +413,15 @@ void RenderView::updateMaterials(mx::TypedElementPtr typedElem)
             }
         }
 
+        // Skip material nodes without upstream shaders.
+        mx::NodePtr node = typedElem ? typedElem->asA<mx::Node>() : nullptr;
+        if (node &&
+            node->getCategory() == mx::SURFACE_MATERIAL_NODE_STRING &&
+            mx::getShaderNodes(node).empty())
+        {
+            typedElem = nullptr;
+        }
+
         mx::TypedElementPtr udimElement = nullptr;
         mx::NodePtr materialNode = nullptr;
         if (typedElem)