Change flex constraints to eigenmodes of the stiffness matrix.

This provides a reduction from 26 to 18 constraints for trilinear and from 162 to 75 for quadratic. The assembly of the constraints becomes trivial. In total the speedup for a trilinear 3x3x3 grid is about 3x.

PiperOrigin-RevId: 902502398
Change-Id: I764772c7adef78da5a644f64701f842d36e4b543

src/user/user_model.cc

@@ -3657,6 +3657,8 @@ void mjCModel::CopyObjects(mjModel* m) {
         int b1 = pfl->vertbodyid[pfl->edge[k].first];
         int b2 = pfl->vertbodyid[pfl->edge[k].second];
         m->flexedge_rigid[edge_adr+k] = (bodies_[b1]->weldid == bodies_[b2]->weldid);
+      } else {
+        m->flexedge_rigid[edge_adr+k] = 0;
       }
     }