Travel to: fix travel to on ramps

[Brambor]

Summary

Bugfixes "Travel to no longer cancels on moving up/down on a ramp"

Purpose of change

Travel to doesn't work with ramps:

Cataclysm_.Dark.Days.Ahead.-.0.G-10289-g31fb5cbf61-dirty.2024-06-24.19-21-45.mp4

Commanding the character to Travel to.

Note the bottom right: The moment you step down it cancels.

Describe the solution

1. Change get_movement_action_from_delta (called only in get_next_auto_move_direction):
   • Consider xy before z, if xy is valid and non-zero, ignore z.
   • If none of the arguments are valid, return ACTION_NULL instead of ACTION_MOVE_FORTH_LEFT.
2. Change get_next_auto_move_direction:
   • Allow the character to be 1 tile off of the expected direction.
   • The off by 1 in z coordinate is needed for moving over ramps.
   • The off by 1 in x and y is likely not going to be used, but if the character wants to move east and is, for some reason, pushed to the south, they can move northeast and be back the path.
   • Every cycle removes one tile from the route, so if the character wants to move, but fails, it now takes two cycles instead of one to detect this. However, it still terminates.
   • This change would allow for the character to be on the wrong floor if something screws up. I am open to suggestions. (See testing)

• Rename var as second commit, because I don't want to make a separate PR.
• Rationale: I am renaming ramp_offset to ramp_z_offset as I didn't know what it was until I got to the last line. It also might have been teleporting in x or y direction, not the z direction.

Describe alternatives you've considered

Change the path generation to know about ramps and produce the path accordingly. I prefer this as this is simple and also fixes ramp movement in my upcoming PR:

• Auto path #74644

Testing

Travel to across ramp now works:

Cataclysm_.Dark.Days.Ahead.-.0.G-10289-g9fcb743a1e-dirty.2024-06-24.19-10-11.mp4

Indeed, the character can walk on the wrong floor, if they automove onto a ramp they didn't expect:

Before (better)

Cataclysm_.Dark.Days.Ahead.-.0.G-10289-g31fb5cbf61-dirty.2024-06-24.19-26-26.mp4

After (worse)

Cataclysm_.Dark.Days.Ahead.-.0.G-10289-g31fb5cbf61-dirty.2024-06-24.19-35-53.mp4

However, I believe this PR is a net benefit.

Overmap travel to still says "You cannot reach the destination" when on such a bridge, but that is likely a different problem.

Additional context

Originally found here: #74644 (comment)