[minor] Schedule overlay on map looks weird on slopes and bridge heads

[Nazalassa]

Minor aesthetic issue: the overlay does not remain on the tracks on slopes and bridge heads.

[prissi]

grund_t::get_weg_yoff() can give the right extra offset, but one has to look up the ground first ...

[victor_18993]

Fixed in r12213.

The schedule route overlay was using the ground base projection and therefore did not follow the actual way surface correctly through slopes and bridge heads.

The fix now uses the existing way-height information and adds the required breakpoint at slope transitions. Flat ground, slopes, bridge heads and all four rotations are covered by regression tests.

Thanks for reporting it.