Slider handling agaaaaaaaaaain

[84634E1A607A]

Brief Description of What This PR Does

This time used a linear algo to calculate the end position. This doesn't rely on the initial value so it should be fine.
Also fixed a bug where float to int cast is directly using (int) instead of Math.Round(), causing strange loss.

Related Issues
Fixes #3639
alleviates #1533 (?) let's leave that open though as it is an issue tracking a Godot bug we are interested in having fixed -hhyyrylainen
Fixes #3203
Closes #3452

Needs test: #3562

Progress Checklist

Note: before starting this checklist the PR should be marked as non-draft.

• PR author has checked that this PR works as intended and doesn't
  break existing features:
  https://wiki.revolutionarygamesstudio.com/wiki/Testing_Checklist
  (this is important as to not waste the time of Thrive team
  members reviewing this PR)
• Initial code review passed (this and further items should not be checked by the PR author)
• Functionality is confirmed working by another person (see above checklist link)
• Final code review is passed and code conforms to the
  styleguide.

Before merging all CI jobs should finish on this PR without errors, if
there are automatically detected style issues they should be fixed by
the PR author. Merging must follow our
styleguide.

[hhyyrylainen]

From a code standpoint this change looks fine to me. Let's wait some more time if we can get someone to playtest this.

[hhyyrylainen]

I did a quick test and this seems to work fine enough so I'll merge this as it doesn't look like anyone else will test this.