Fix #10164: Incorrect slider handle position in RTL.

[PeterN]

Motivation / Problem

As per #10164, the slider widget handle is incorrectly positioned in RTL.

Description

This is because max_value already had min_value removed to get the slider's range. Therefore min_value needs to be taken from value before adjusting for RTL.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker