…. And starting from depot tiles.

Motivation / Problem

When laying track using the AutoRail tool, I often find myself accidentally starting on a bridge, tunnel or station tile. This invalidates the entire action and no rail is built. The other way around, i.e. starting at an empty tile and ending on say a station tile, that works just fine. This can be quite annoying for example when you are trying to create lots of bridges / tunnels in a crowded map.

Description

My changes allow the user to start the AutoRail tool on train station tiles, train depot tiles, bridgehead tiles and tunnel entrance tiles. The user must drag in the right direction, or can be 45 degrees off.

In addition, the user can drag "through" a tunnel, bridge or station. This obviously doesn't apply to depots. If there are multiple tunnels or a tunnel + bridge above each other, we would follow the route that a train would take when driving along the dragged line. In other words, we simply enter the first tunnel or bridge, and exit out of the other end.

[EDIT] This now only applies when one is creating new track. When deleting track, basically we just "delete whatever we can". This approach is more in line with how we currently do things (before my changes).

Here's an impression of what all of this looks like:

[EDIT] And now also for roads/trams:

Limitations

I like the visualization, but I need to jump through a number of hoops to get there. Worst case I can ditch the new visualization.

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