[feat]The mechanisms for collisions between a finite mass cube and other bodies are not properly implemented.

[chenzunh23]

Having added several cubes with finite mass or gravity in game_ball.cpp, I found that they would always sink into the playground (a cube with infinite mass) once a collision with a sphere has occured. What is even weirder is that those with gravity would simply sink beneath the playground without any collision. I understand that the collisions between cubes are complex, though.