Refactor hfield-convex collision code. This should be a no-op.

PiperOrigin-RevId: 837045492
Change-Id: I82851615c7f69774da83cab361316e5a6e9982c6

src/engine/engine_derivative.c

@@ -247,7 +247,7 @@ void mjd_subQuat(const mjtNum qa[4], const mjtNum qb[4], mjtNum Da[9], mjtNum Db
   mju_addToScl(Da_tmp, KK, coef, 9);
 
   if (Da) {
-    mju_copy(Da, Da_tmp, 9);
+    mju_copy9(Da, Da_tmp);
   }
 
   if (Db) {  // Db = -Da^T
@@ -309,7 +309,7 @@ void mjd_quatIntegrate(const mjtNum vel[3], mjtNum scale,
     if (Dquat)          Dquat[i] = a*eye[i] + b*cross[i] + c*outer[i];
     if (Dvel || Dscale) Dvel_[i] = b*eye[i] + c*cross[i] + d*outer[i];
   }
-  if (Dvel) mju_copy(Dvel, Dvel_, 9);
+  if (Dvel) mju_copy9(Dvel, Dvel_);
   if (Dscale) mju_mulMatVec3(Dscale, Dvel_, vel);
 }