Change: Allow overbuilding station and waypoint tiles

[2TallTyler]

Motivation / Problem

#9852 made overbuilding free, but blocked overbuilding without changing any tiles. This is very common when building stations using NewGRF station sets, where platforms can have different canopies, footbridges, etc., so having it blocked is frustrating.

Description

It doesn't hurt anybody to allow overbuilding, even if graphics are the same, so I just removed the error entirely. Costs are still calculated correctly, which was the point of #9852.

Rotating road stations is still only possible for bay stations, since drive-through stations will throw an error about the underlying road.

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

[FLHerne]

It's explicitly intended by the author of #9852, so the commit message should perhaps be 'Change' rather than 'Fix'. I suspect it (or its consequences for custom stations) may have been overlooked by reviewers though.

If no new rail stations are built or rotated, the command returns a STR_ERROR_ALREADY_BUILT error.
If there already is a rail waypoint, the command returns a STR_ERROR_ALREADY_BUILT error.

I agree that allowing overbuilding is preferable, the current behaviour would be very inconvenient for building NewGRF stations.

[2TallTyler]

Ah, good point. Commit title changed.

This will need backporting to 13.1 or we'll get tons of bug reports if we release without it. 😉

[glx22]

No need to backport, the original change is only in master.