Diagonal reversing bug & fix

[freddyhayward]

Convoys are currently forced to unnecessarily reverse when having their schedules edited on diagonal ways, even when no changes have been made, or when the new route does not require a reversal. This pull request fixes the issue: https://github.com/jamespetts/simutrans-extended/pull/217

The problem occurs because the facing direction of a convoy on a diagonal tile is always a compound direction (e.g. northeast, southwest) where the next tile in the route is technically only accessible from a single direction (e.g. north, south).

[jamespetts]

Excellent, thank you for that: now incorporated.