Fix: link overlay line width on HiDPI displays

[Bouke]

Motivation / Problem

On HiDPI displays we draw the game at double resolution (GUI scale) to get a consistent physical GUI size. The link graph overlay uses a hard-coded line width of 3 pixels which is not adjusted for GUI scale. This makes the overlay's lines difficult to see, especially to distinguish the different saturation colours. Similar to #9997 this now uses the GUI scaling to translate these 3px to whatever is appropriate for the current GUI scaling level.

Description

Expected:

Actual (12.2):

Fix:

Limitations

LinkGraphOverlay is initialized on map load. As the GUI scale can be changed in-game, we need to postpone scale the line width to the latest moment possible: on draw.

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