Rebase ontop of origin/main

Added usd_reader_pointinstancer.h to CMakeLists.

Creating readers for point instancer prototypes.

Creating collections containing point instancer prototype objects.

Finished setup of the point cloud geometry nodes modifier node tree,
as follows: Creating Named Attribute nodes for reading the
"orientations", "scales" and "proto_indices" point attributes;
setting the required flags on the Collection Info and Instance On
Points nodes; setting the prototype collection object on the
Collection Info node.

Added a USDPrimReader::is_in_instancer_proto_ boolean flag which
identifies readers of prims that are in point instancer prototypes.
The flag simplifies management of collections and is used to
determine whether global transforms should be applied to root objects.

Fixed formatting.

@@ -0,0 +78,4 @@
+        positions_span[i] = float3(positions[i][0], positions[i][1], positions[i][2]);
+    }
+
+    auto scales_attribute = point_cloud->attributes_for_write().lookup_or_add_for_write_only_span<float3>("scales", ATTR_DOMAIN_POINT);

@@ -0,0 +153,4 @@
+    collection_info_node->locx = 100.0f;
+    collection_info_node->locy = 100.0f;
+    bool seperate_children = true;
+    nodeFindSocket(collection_info_node, SOCK_IN, "Separate Children")->default_value = (void*)&seperate_children;

@@ -0,0 +38,4 @@
+/**
+ * Create a node to read a geometry attribute of the given name and type.
+ */
+static bNode *add_input_named_attrib_node(bNodeTree *ntree, const char *name, int8_t prop_type)

@@ -0,0 +107,4 @@
+
+  PointCloud *point_cloud = BKE_pointcloud_new_nomain(positions.size());
+
+  auto positions_span = point_cloud->positions_for_write();

@@ -0,0 +114,4 @@
+  }
+
+  auto scales_attribute =
+      point_cloud->attributes_for_write().lookup_or_add_for_write_only_span<float3>(

@@ -0,0 +117,4 @@
+      point_cloud->attributes_for_write().lookup_or_add_for_write_only_span<float3>(
+          "scales", bke::AttrDomain::Point);
+
+  for (int i = 0; i < positions.size(); i++) {

@@ -0,0 +140,4 @@
+                                                        orientations[i].GetImaginary()[2]);
+    }
+    else {
+      orientations_attribute.span[i] = math::Quaternion();

@@ -0,0 +161,4 @@
+  ModifierData *md = BKE_modifier_new(eModifierType_Nodes);
+  BLI_addtail(&object_->modifiers, md);
+  NodesModifierData &nmd = *reinterpret_cast<NodesModifierData *>(md);
+  nmd.node_group = ntreeAddTree(NULL, "Instances", "GeometryNodeTree");

@@ -0,0 +191,4 @@
+  indices_attrib_node->locx = 100.0f;
+  indices_attrib_node->locy = -300.0f;
+
+  bNode *rotation_to_euler_node = nodeAddStaticNode(NULL, ntree, FN_NODE_ROTATION_TO_EULER);

@@ -0,0 +268,4 @@
+
+void USDPointInstancerReader::set_collection(Main *bmain, Collection *coll)
+{
+  if (!object_) {

@@ -0,0 +274,4 @@
+
+  BLI_assert(coll);
+
+  ModifierData *mod_data = BKE_modifiers_findby_type(this->object_, eModifierType_Nodes);

@@ -0,0 +281,4 @@
+  }
+
+  NodesModifierData *nmd = reinterpret_cast<NodesModifierData *>(mod_data);
+  if (!nmd) {

@@ -0,0 +304,4 @@
+    return;
+  }
+
+  bNodeSocketValueCollection *socket_data = (bNodeSocketValueCollection *)sock->default_value;

@@ -0,0 +1,57 @@
+/*

@@ -54,10 +56,25 @@
 
 #include "WM_api.hh"
 
+#include <iostream>

@@ -618,0 +706,4 @@
+      continue;
+    }
+
+    blender::Vector<USDPrimReader *> proto_readers;

@@ -0,0 +163,4 @@
+  bNode *instance_on_points_node = nodeAddStaticNode(nullptr, ntree, GEO_NODE_INSTANCE_ON_POINTS);
+  instance_on_points_node->locx = 300.0f;
+  bNodeSocket *socket = nodeFindSocket(instance_on_points_node, SOCK_IN, "Pick Instance");
+  ((bNodeSocketValueBoolean *)socket->default_value)->value = true;

@@ -0,0 +245,4 @@
+              group_output,
+              static_cast<bNodeSocket *>(group_output->inputs.first));
+
+  BKE_ntree_update_main_tree(bmain, ntree, nullptr);

@@ -618,0 +718,4 @@
+  }
+}
+
+std::optional<UsdPathSet> USDStageReader::collect_point_instancer_proto_paths() const

@@ -618,0 +700,4 @@
+      /* Format the collection name to follow the proto_<index> pattern. */
+      std::ostringstream ss;
+      ss << std::setw(max_index_digits) << std::setfill('0') << proto_index;
+      std::string coll_name = "proto_" + ss.str();

@@ -117,0 +139,4 @@
+   *          the prim cannot be converted.
+   */
+  USDPrimReader *collect_readers(const pxr::UsdPrim &prim,
+                                 const UsdPathSet *pruned_prims,