Flag functions instead of defining glyphs directly (issue4625067)

[lemniskata . bernoullego]

Reviewers: carl.d.sorensen_gmail.com, MikeSol,

Message:
This is an intermediate step that will be needed for shortened flags.

Description:
Flag functions instead of defining glyphs directly

We will need many length variants of every flag.
Therefore instead of writing flag code directly
in glyph definition, it should be written as
a function and called back later as appropriate.
The argument shortening is the amount the flag
should be shorter than default.
As for now it is used in a very primitive way,
only to demonstrate the shortening effect
and to test c++ code on something.

Please review this at http://codereview.appspot.com/4625067/

Affected files:
  M mf/feta-flags.mf