in what unit is shift_amount measured? (preparing fix for 1546)

[Janek Warchoł]

Hi,

i'm pretty confused about how note columns shift is measured (line 277
and following in note-collision.cc).  I tried modifying the values
there, but the results were quite unexpected; i tried to trace in what
unit is shift_amount measured but to no avail.  Could you enlighten
me?
(my idea to fix 1546 is to include X-extent of the left note column in
shift_amount).

cheers,
Janek