* lily/slur.cc (outside_slur_callback): make offset_scm optional.

* lily/slur.cc (outside_slur_callback): make offset_scm optional.

* lily/beam.cc (rest_collision_callback): make prev_offset optional.

* lily/grob-closure.cc (chain_offset_callback): don't pass 0 but
SCM_UNDEFINED for non-existent data.

* lily/side-position-interface.cc (general_side_position): allow
optional current_offset argument for chaining, so combinations of side-position
and outside slur callback don't add up.  Fixes issue #92.