support obj with negative indices

editor/import/resource_importer_obj.cpp

@@ -168,18 +168,23 @@ Error ResourceImporterOBJ::import(const String &p_source_file, const String &p_s
 
 					if (face[idx].size() == 3) {
 						int norm = face[idx][2].to_int() - 1;
+						if (norm < 0)
+							norm += normals.size() + 1;
 						ERR_FAIL_INDEX_V(norm, normals.size(), ERR_PARSE_ERROR);
 						surf_tool->add_normal(normals[norm]);
 					}
 
 					if (face[idx].size() >= 2 && face[idx][1] != String()) {
-
 						int uv = face[idx][1].to_int() - 1;
+						if (uv < 0)
+							uv += uvs.size() + 1;
 						ERR_FAIL_INDEX_V(uv, uvs.size(), ERR_PARSE_ERROR);
 						surf_tool->add_uv(uvs[uv]);
 					}
 
 					int vtx = face[idx][0].to_int() - 1;
+					if (vtx < 0)
+						vtx += vertices.size() + 1;
 					ERR_FAIL_INDEX_V(vtx, vertices.size(), ERR_PARSE_ERROR);
 
 					Vector3 vertex = vertices[vtx];