Pong demo: Fix ball velocity decay

In moddable pong the project had linear damp and angular damp in zero.
To do that only for the ball (not project wide) we can set linear and
angular damp mode to "replace" in the RigidBody2D.

pong_game/ball.tscn

@@ -20,6 +20,8 @@ continuous_cd = 1
 max_contacts_reported = 1
 contact_monitor = true
 linear_velocity = Vector2(353.553, 353.553)
+linear_damp_mode = 1
+angular_damp_mode = 1
 script = ExtResource("1_vdrab")
 
 [node name="CollisionShape2D" type="CollisionShape2D" parent="."]