Unifies mensural ligatures with blot-diameter. (issue 5030053)

[bordage . bertrand]

Reviewers: benko.pal,

Message:
Hi,

The new mensural style introduced with commit
0dcc93c0a5a97d048db2f7631966f41ae0059ab5 created some ugliness's in
mensural ligatures.

This patch fix these. I also added serifs to flexas.

Regards,
Bertrand

Description:
Unifies mensural ligatures with blot-diameter.

Reverts some changes of commit 0dcc93c0a5a97d048db2f7631966f41ae0059ab5

Use blot-diameter either in C and MetaFont.

Please review this at http://codereview.appspot.com/5030053/

Affected files:
  M lily/mensural-ligature.cc
  M mf/parmesan-noteheads.mf