Fix #10257: Incorrect wire position on sloped bridge heads.

[PeterN]

Motivation / Problem

See #10257. El-rail wire position is incorrect on south-facing sloped bridge heads.

Description

The cause of this is alluded to in the comments:

Also note that the result of GetSlopePixelZ() is very special for bridge-ramps

This PR resolves this issue by rounding the z value to the nearest whole height-level increment, and adjusting z-offsets as appropriate.

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, 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