Rigidity slider fix

[Patryk26g]

Brief Description of What This PR Does

Hopefully this fixes the rigidity slider issue. The problem was mainly with the division by 0.

Related Issues

Closes #4150

Progress Checklist

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[dligr]

I have found a new corner case:

1. Set mutation cost at 1.9
2. Place two cytoplasms
3. Drag the rigidity slider to get the MP to 7
4. Skip two slider steps with a mouse press; the MP ends up at -1

It seems like the issue is that the step cutoff calculations don't take into account the extra 1 that may be added due to rounding

[Patryk26g]

I think this corner case is working properly now