[m17n-list] Adding a Font Layout Table

[Richard Wordingham]

I'm trying to add a font layout table (FLT) for OpenType Layout fonts
(well, that use the new HarfBuzz interpretation) for the Tai Tham
script.  My aim is to use it for Emacs.

Is there an easy way of debugging such a layout?  It seems that I cannot
induce Emacs to use the FLT, and I don't know why.  I am running Emacs
with the Bash shell command

MDEBUG_ALL=1 M17NDIR=/home/richard/tham/m17n/lib emacs

I get debug messages telling me that the new FLT has been loaded

[DB]  <font,layouter,lana-otf,nil>.

but no hint that shaping is being applied to the font.  I have added
two hooks:

In truetype.fst:

(tai_tham
 (nil
  ((nil nil unicode-bmp :otf=lana) lana-otf)))

In SCRIPT.tbl:

(tai_tham (nod khh khb lo tts pi) (#x1A20 #x1A53) lana)

Should these hooks suffice from the m17n side?  The Tai Tham font that
gets used, Da Lekh, has, for the script lana, GSUB features abvs, blwf,
blws, ccmp, clig, liga, pref, pstf, rlig and ss01 and GPOS features
mark and mkmk.

For Thai characters, the same run will invoke what is recorded as
(thai-otf (norasi)).

Richard.

Richard.