Plane: more fixes for glide slope calculations
Plane: more accurate altitude tracking close to waypoints

don't use a glide slope if the altitude change is less than 15 meters
Plane: added GLIDE_SLOPE_MIN parameter

this is used to prevent slow glide slopes being used for small
altitude changes in missions. This allows more accurate tracking of
altitude with terrain changes
Plane: change threshold for no glide slope to 20m

40m is quite high for most planes
Plane: remved int cast

not portable
Plane: use bitfields in auto_state
Plane: prevent cross-track on some WP types

when finishing a takeoff or when a user forces a WP change don't do
any cross-tracking. This avoids a problem where the plane tries to
line up with a track completely unrelated to the track currently being
flown