It was due to main_shape_changed being called two times for each
added body. The first time it disables ccd, which sets the internal ccd
threshold to be 10000. The second time, it enables ccd again because
the internal threshold is > 0.
Bodies are now consistently set with a ccd threshold of 0 when ccd is
disabled.
This was causing crashing asserts in Bullet when adding bodies in some
scenarios, in btVector3::normalize():
btAssert(!fuzzyZero());
These crashes will still happen with ccd enabled.
