[m17n-list] Adding a Font Layout Table

[Richard Wordingham]

I've now got Emacs to use my FLT for Tai Tham - the trick was to add
entries to the Emacs char table composition-function-table.

While the FLT works for my Tai Tham font, I want it to also work for
fonts designed to work with Microsoft's new Universal Shaping Engine.
The problem I have is that several GSUB features are invoked before
Indic shaping:

  locl, ccmp, akhn, nukt

and, to a lesser extent, some features are invoked to control the
shaping itself:

  pref, rphf (TBC)

My question is then, what happens to the 'category' as defined by
the category table when features are applied?  I'm not doing a good job
of reading and understanding m17n-flt.c.  Is the behaviour documented
anywhere?

From reading MLM2-OTF.flt in the m17n database, it seems that it is
possible to specify a category for the glyphs changed by a feature.
Experimentation reveals that 'changed' includes null changes that
replace a glyph by itself.  That is in general good for Tai Tham, but
it does mean that my work-around for libotf bug
https://savannah.nongnu.org/bugs/?44584 (objection to a contextual
substitution string invoking no substitutions) may not work for such
features.

On the other hand, if there is no category defined for a feature, a
null change does not change the category, but any other single
substitution does seem to change the category.  This is  behaviour I
would want to override for the feature locl.

Richard.