Fix non-determinism in XGL importer output due to std::multimap value ordering not guaranteed < C++11.

One step closer to a fully reproducible regression test suite.

code/XGLLoader.cpp

@@ -431,6 +431,12 @@ aiNode* XGLImporter::ReadObject(TempScope& scope, bool skipFirst, const char* cl
 		throw;
 	}
 
+	// FIX: since we used std::multimap<> to keep meshes by id, mesh order now depends on the behaviour
+	// of the multimap implementation with respect to the ordering of entries with same values.
+	// C++11 gives the guarantee that it uses insertion order, before it is implementation-specific.
+	// Sort by material id to always guarantee a deterministic result.
+	std::sort(meshes.begin(), meshes.end(), SortMeshByMaterialId(scope));
+
 	// link meshes to node
 	nd->mNumMeshes = static_cast<unsigned int>(meshes.size());
 	if (nd->mNumMeshes) {