Feature: Add font resizing sliders to the game options UI.

[ladysadie]

Motivation / Problem

The OpentTTD's small font (FS_SMALL) can be illegible on high resolution displays even after the interface scaling is applied.

Description

Sliders have been added to allow the player to easily adjust the font sizes of the UI. The prefer sprite and AA options were moved from the graphics tab to the new font since they're a font related setting.

The new UI is pictured below:

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

[ladysadie]

Should I add the header guards for HAS_TRUETYPE_FONT around the font tab UI code? I considered doing it. Wasn't sure if the "juice was worth the squeeze" for it though.