Wrong slope force calculation for flipped vehicles

[xmnovotny]

When vehicle is flipped, slope force (and acceleration/power needed) is calculated as on opposite direction - going downhill will slow down the vehicle and vice versa.

Steps to reproduce
Build long slope, long doubleheaded train and observe speed, acceleration and power needed for going uphill and downhill after whole train reverse its direction.

Build version
0.86.1.0

Platform
Windows 10

Reproducibility
Everytime

Fix
In function Vehicle.GetSlopeForce multiply whole result by -1 when Vehicle.IsFlipped and subresults by -1 when VehicleUnit.IsFlipped

[andrewpey]

Thanks, will be fixed with 0.87.

[andrewpey]

The fix is live on the beta branch.