OpenSubdiv: Allow less topology callbacks assigned

Useful for cases when topology does not need to have any
crease or UV layers. Now instead of assigning callbacks
which returns zero data is possible to simply assign the
callback itself to NULL.

intern/opensubdiv/internal/opensubdiv_converter_factory.cc

@@ -140,6 +140,12 @@ inline bool TopologyRefinerFactory<TopologyRefinerData>::assignComponentTags(
   using OpenSubdiv::Sdc::Crease;
   const OpenSubdiv_Converter *converter = cb_data.converter;
   const bool full_topology_specified = converter->specifiesFullTopology(converter);
+  if (!full_topology_specified && converter->getNumEdges == NULL) {
+    assert(converter->getEdgeSharpness == NULL);
+    assert(converter->getVertexSharpness == NULL);
+    assert(converter->isInfiniteSharpVertex == NULL);
+    return true;
+  }
   const int num_edges = converter->getNumEdges(converter);
   for (int edge_index = 0; edge_index < num_edges; ++edge_index) {
     const float sharpness = converter->getEdgeSharpness(converter, edge_index);
@@ -190,6 +196,13 @@ inline bool TopologyRefinerFactory<TopologyRefinerData>::assignFaceVaryingTopolo
     TopologyRefiner &refiner, const TopologyRefinerData &cb_data)
 {
   const OpenSubdiv_Converter *converter = cb_data.converter;
+  if (converter->getNumUVLayers == NULL) {
+    assert(converter->precalcUVLayer == NULL);
+    assert(converter->getNumUVCoordinates == NULL);
+    assert(converter->getFaceCornerUVIndex == NULL);
+    assert(converter->finishUVLayer == NULL);
+    return true;
+  }
   const int num_layers = converter->getNumUVLayers(converter);
   if (num_layers <= 0) {
     // No UV maps, we can skip any face-varying data.