WayPoint Glide Slope Angle Calculations are Wrong Sometimes

#### Issue details
The calculated "Angle" in the flight planing table is wrong some times. This is critical for Fixed wing landing plans.

This occurs when I have distance and altitude units configured for feet, I believe this issue is likely related to unit conversion.

The screenshot below shows a glide slope of three degrees with a 400Ft descent over 2300 ft. The angle should report -9.8 degrees here


#### Version
1.3.58

Start by reproducing the issue in Mission Planner 1.3.58 with distance and altitude configured in feet, using a 400 ft descent over 2300 ft in a fixed-wing landing plan. Trace the waypoint glide-slope angle calculation and its unit conversion; done means the example reports -9.8 degrees and equivalent configurations remain correct.

Written by the indexing model from the issue text.