Slopes speed modifier miscalculation

MoveMath uses average slopes for the speed modifier, but individual triangles' normals to check for downward/upward.

This means that on a steep upward slope a unit could get a movespeed increases if a single triangle is downward

https://github.com/beyond-all-reason/spring/blob/d69fb498bed99f075f37df480ea970ceb3c7c686/rts/Sim/MoveTypes/MoveMath/MoveMath.cpp#L82

https://github.com/beyond-all-reason/spring/blob/d69fb498bed99f075f37df480ea970ceb3c7c686/rts/Sim/MoveTypes/MoveMath/MoveMath.cpp#L86

Start in rts/Sim/MoveTypes/MoveMath/MoveMath.cpp at the linked lines 82 and 86. Trace how average slopes and individual triangle normals determine the speed modifier, then verify the behavior on a steep upward slope. Done means the modifier no longer increases because of a single downward-facing triangle.

Written by the indexing model from the issue text.