Fix: USD instancing export errors

Fixed two errors when exporting to USD with Instancing enabled:

Ensuring the mesh prototype prim exists before referencing it to
avoid the "Unresolved reference prim path" error messages in the
console.

Adding the Root Prim path prefix to the prototype reference path.

Pull Request: https://projects.blender.org/blender/blender/pulls/126210

source/blender/io/usd/intern/usd_writer_abstract.cc

@@ -261,7 +261,17 @@ bool USDAbstractWriter::mark_as_instance(const HierarchyContext &context, const
     return false;
   }
 
-  pxr::SdfPath ref_path(context.original_export_path);
+  BLI_assert(!context.original_export_path.empty());
+  BLI_assert(context.original_export_path.front() == '/');
+
+  std::string ref_path_str(usd_export_context_.export_params.root_prim_path);
+  ref_path_str += context.original_export_path;
+
+  pxr::SdfPath ref_path(ref_path_str);
+
+  /* To avoid USD errors, make sure the referenced path exists. */
+  usd_export_context_.stage->DefinePrim(ref_path);
+
   if (!prim.GetReferences().AddInternalReference(ref_path)) {
     /* See this URL for a description for why referencing may fail"
      * https://graphics.pixar.com/usd/docs/api/class_usd_references.html#Usd_Failing_References