The during trim-front, the call order is
1. Region::set_position_internal
2. Region::set_length_internal
::set_position_internal sets `_last_length.set_position (position());`
but the later call ::set_length_internal does `_last_length = _length;`
This sets last_position() == position(), and later Region::mid_thaw()
does not call Region::recompute_at_start().
Region gain envelope was not updated.
see also 00a1b7cb24 and 13d10582f0
8ba21e7fff