Refactor Default class to use pointers instead of indices.

This in order to make it easier to append the default tree to another tree.

PiperOrigin-RevId: 649812975
Change-Id: I2915ac7e37957e45eadc24d6caaf7beea236bf29

src/user/user_mesh.cc

@@ -149,7 +149,7 @@ mjCMesh::mjCMesh(mjCModel* _model, mjCDef* _def) {
 
   // set model, def
   model = _model;
-  def = (_def ? _def : (_model ? _model->Defaults(0) : 0));
+  def = (_def ? _def : (_model ? _model->Default() : 0));
 
   // in case this body is not compiled
   CopyFromSpec();