Allows seconds to merge (issue 5729057)

[dak]

http://codereview.appspot.com/5729057/diff/1/lily/note-collision.cc
File lily/note-collision.cc (right):

http://codereview.appspot.com/5729057/diff/1/lily/note-collision.cc#newcode131
lily/note-collision.cc:131: min_downs = min (min_ups, dps[i][X_AXIS]);
min (min_ups, ...
Seriously?

And it is nonsensical to compare the mins on either side, since for the
stem-down configuration, the max is the part to merge.

http://codereview.appspot.com/5729057/diff/1/lily/note-collision.cc#newcode139
lily/note-collision.cc:139: }
This assumes that only mins/maxes need to be considered when merging
when in reality it is every pairing that has a chance to overlap.  It is
just that the min/max pairing with the smallest distance determines
which stem distance has a chance of succeeding, and once one has
determined that, one needs to do a merge-like run through both columns
to make sure that at this stem distance, all note heads either merge to
identical positions, or don't touch one another.

http://codereview.appspot.com/5729057/