/*  ****************** 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)
  
  *************************************************************************** */