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/engine/engine_derivative.c

@@ -906,6 +906,11 @@ static void mjd_flexInterp_kernel(const mjModel* m, mjData* d, mjtFlexOp op,
       continue;
     }
 
+    // skip if strain constraints present (stiffness handled by constraint solver)
+    if (m->flex_edgeequality[f] == 3) {
+      continue;
+    }
+
     // compute scale
     mjtNum damping = m->flex_damping[f];
     mjtNum scale = s1 + s2 * damping;