Fix: Layouter reverse-mapping to characters

[nielsmh]

Motivation / Problem

Mapping back from a glyph to a character index in the source string should work, even with multiple run and when formatting codes are involved.

Description

Have visual runs keep track of their offsets in the input text.
Count formatting codes the same way in both forward and reverse mapping.

Limitations

Not thoroughly tested recently, but this fixed something when I was working on #7786 originally. (Reverse-mapping is relevant for hit-testing mouse clicks against individual character glyphs.)

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