Rigidity slider movement when there is not enough MP can end up with 1 or 2 MP remaining (instead of 0)

[hhyyrylainen]

After this bugfix: #5778 the slider can now when trying to move for example 16 MP worth, but when there's only 8 MP left, it uses either 6 or 7 MP (so leaves the player with 1 when the bar is clicked too far). Instead it would be preferable if the slider went as far as it can in a single step (so until there's 0 MP left, not until there's 1 MP left).