Motivation / Problem

Buried in the text & savegame for #8594 was mention of order speed limits. The number of people who know about & use this feature is probably vanishingly small, but it seems proper that the pathfinder should take it into account when calculating the best route.

Description

Set the vehicle's max speed in the pathfinder. I could've set this in GetSpeedLimit in follow_track.hpp, but this seemed otherwise unrelated to the "track".
Order speed for road vehicles doubled because of road veh units (and that's what's done elsewhere)

Limitations

More possible changes to routing behaviour - but if you're using order speed limits, you're already doing some pretty custom stuff.

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