ledger-line-spanner.cc: refactor Ledger_line_spanner::print (issue 297820043 by address@hidden)

[paulwmorris]

Reviewers: ,

Message:
Please review.  I was working on a scheme version of the ledger line
print routine (for some custom ledger line patterns) and having figuring
out how it worked, I thought I would make some improvements.

Since I'm new to C++ ...review is especially appreciated.

Thanks,
-Paul

Description:
ledger-line-spanner.cc: refactor Ledger_line_spanner::print

Optimizations and removal of code duplication.

Only iterate through all note heads once, generating
all relevant data for just note heads involved in
ledger lines.  Then work with that data.

Merge overlapping ledger data before producing
ledger stencils so we only produce one stencil for
each ledger line.

Please review this at https://codereview.appspot.com/297820043/

Affected files (+142, -81 lines):
  M lily/ledger-line-spanner.cc