SpeedModifiers calculated from slopes are inverted when unit is moving in reverse

https://github.com/beyond-all-reason/RecoilEngine/blob/391f1fbf06f69d3ebc32e31b50fc9c65f45da0a1/rts/Sim/MoveTypes/GroundMoveType.cpp#L1298

Seems to use flatFrontDir as the "movement" vector when calculating slope mod, rather than the movement direction itself.
Should use something along the lines of a (-1)*flatFrontDir vector.
This currently results in units accelerating when climbing in reverse, and decelerating when going downhill.

Start at rts/Sim/MoveTypes/GroundMoveType.cpp around line 1298, where the slope modifier is calculated. Compare the vector used there with the unit's actual movement direction and inspect the flatFrontDir handling. Done means reverse movement applies the correct acceleration or deceleration when climbing and descending slopes.

Written by the indexing model from the issue text.