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

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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

[dligr]

I have found another corner case:

1. Set mutation cost at 0.2
2. Get to editor
3. Place some hydrogenases to get MP to 1
4. Move the rigidity slider three steps and it becomes immovable

[Patryk26g]

I have found another corner case:

I took care of it as well

[dligr]

The only issue I could find now is that when choosing rigidity though clicking while having low enough MP, there might be more steps cut off than needed.
For example, with the MP cost modifier of 1.9:

1. Switch to any membrane type
2. Move rigidity one step left
3. Click somewhere on the right part of the slider

The slider's value would then be one step away from the right value due to excessive cut off

[Patryk26g]

The only issue I could find now is that when choosing rigidity though clicking while having low enough MP, there might be more steps cut off than needed. For example, with the MP cost modifier of 1.9:

1. Switch to any membrane type
2. Move rigidity one step left
3. Click somewhere on the right part of the slider

The slider's value would then be one step away from the right value due to excessive cut off

Well that seems like a really rare case and not that problematic. I think that in order to cover this one I would actually need to rewrite the whole function from scratch because I don't see any other solution

[hhyyrylainen]

This fixes the primary bug but as @dligr found out this leaves 1 or 2 MP when trying to move too far in one step, which is a pretty noticeable bug when playing with the slider.

As this has been open for a while and fixes the game locking up, I'll merge this but open a follow up issue.

[hhyyrylainen]

Opened the new issue: #5821