Add internal functions for 3x3 matmul.

PiperOrigin-RevId: 616749614
Change-Id: I7f3ac2ffc826076b6c97424324f03ecd9ccd4655

src/engine/engine_collision_convex.c

@@ -318,7 +318,7 @@ static void mju_rotateFrame(const mjtNum origin[3], const mjtNum rot[9],
   mjtNum mat[9], vec[3], rel[3];
 
   // rotate frame: xmat = rot*xmat
-  mju_mulMatMat(mat, rot, xmat, 3, 3, 3);
+  mju_mulMatMat3(mat, rot, xmat);
   mju_copy(xmat, mat, 9);
 
   // vector to rotation origin: rel = origin - xpos
@@ -682,7 +682,7 @@ int mjc_ConvexHField(const mjModel* m, const mjData* d,
   }
 
   // express geom2 mat in heightfield frame
-  mju_mulMatTMat(mat, mat1, mat2, 3, 3, 3);
+  mju_mulMatTMat3(mat, mat1, mat2);
 
   //------------------------------------- AABB computation, box-box test