/* ****************** And how it works! ****************************************
This is the bone transformation trick; they're hierarchical so each bone(b)
is in the coord system of bone(b-1):
arm_mat(b)= arm_mat(b-1) * yoffs(b-1) * d_root(b) * bone_mat(b)
-> yoffs is just the y axis translation in parent's coord system
-> d_root is the translation of the bone root, also in parent's coord system
pose_mat(b)= pose_mat(b-1) * yoffs(b-1) * d_root(b) * bone_mat(b) * chan_mat(b)
we then - in init deform - store the deform in chan_mat, such that:
pose_mat(b)= arm_mat(b) * chan_mat(b)
*************************************************************************** */