Refactor: separate buffer indexing and swizzling.

This also fixes a sensor data reading bug: previously, _sensorIndex was multiplied by 3 by MjEngineTool.

PiperOrigin-RevId: 451699887
Change-Id: I864bcbf8159857764854b25311a26ec5672125d9

unity/Editor/Components/MjMouseSpring.cs

@@ -159,7 +159,8 @@ namespace Mujoco {
               MujocoLib.mj_objectVelocity(
                   scene.Model, scene.Data, (int)MujocoLib.mjtObj.mjOBJ_BODY, body.MujocoId, res, 0);
               // linear velocity is in the last 3 entries
-              bodyVel = MjEngineTool.UnityVector3(res, 1);
+              bodyVel = MjEngineTool.UnityVector3(
+                  MjEngineTool.MjVector3AtEntry(res, 1));
             }
 
             float springStiffness = 100;