Codefix: Use widget resize step instead of window resize step.

[PeterN]

Motivation / Problem

Build Picker class list filter could be drawn incorrectly depending on how the window can be resized.

This does not affect the current picker windows.

Description

Use the class list widget's resize step (which is always correct) instead of the window resize step (which may not be...)

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, game_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