Currently all force effectors can only act on cloth when the force is
perpendicular to the surface. This makes sense for wind, but not for
other forces; and the user may want even wind to have some friction.
This changes effector code to output two force vectors - although
you of course can pass the same pointer for both. The force is split
between the two outputs based on a new per-effector setting.
Differential Revision: https://developer.blender.org/D8017
9e70129952