*\cSPLINE train

The forklift func_train and viper model_train have BOTH the ROTATE and ROTATE_CONSTANT
spawnflags set, which is translated internally to mean that a spline path is
continuously calculated. The spline is determined by the angles and origin of the
path_corners that the train is currently between.

Yaw_speed and pitch_speed are irrelevant in this version, since the train will always
turn to face its current movement direction.