Fixes #8711.

Motivation / Problem

Manually changing your config makes the game go boom!

Specially: set gui_zoom to 0 and zoom_min to 1. This brings gui_zoom out of the valid range, creating really weird effects.

Description

gui_zoom was never clamp'd between zoom_min/zoom_max.

zoom_min controls how zoomed-in we load sprites. For a value of 1,
no quad-sizes sprites are loaded. If gui_zoom would be 0, meaning
it wants quad-sized sprites to display, it was printing random
stuff to the screen, which could or could not result in crashes.

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 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