Replaced the previous implementation for backface collision handling (in
test_axis function from SAT algorithm) with much simpler logic in the
collision generation phase with face shapes, in order to get rid of
wrong contacts when backface collision is disabled.
Now it just ignores the generated collision if the contact normal is
against the face normal, with a threshold to keep edge contacts.
Added a special case for soft bodies to invert the collision instead of
ignoring it, because for now it's the best solution to avoid soft bodies
to go through concave shapes (they use small spheres). This might be
replaced with a better algorithm for soft bodies later.
