[HarfBuzz] Tai Tham Shaper in HarfBuzz

[Ed]

Hi, Behdad and other HB contributors,

As some of you know, I'm working on creating a Unicode font for the
Tai Tham script which is one of the many Brāhmī-derived abugidas of
Southeast Asia (our project URL: http://hariphunchai.unifont.org/ ).

Like other Indic scripts, Tai Tham shaping is complex and requires
reordering of certain glyphs.  Currently, no operating system provides
support for Tai Tham re-ordering.  However Tai Tham's re-ordering
requirements are much simpler than (for example) those Devanagari.

As it seems that HarfBuzz is now reaching a more mature state, I am
hoping that now is a good time to bring up the idea of having HB
support Tai Tham re-ordering.

Here, in general, is what is required:

(1)  U+1A55 TAI THAM CONSONANT SIGN MEDIAL RA needs to be reordered to
appear *before* a consonant cluster (CC):

                                    LOGICAL BACKSTORE => VISUAL LAYOUT
                                    <CC> + <U+1A55>     =>     <U+1A55><CC>

(2) The following five Prefixed Dependent Vowel (PDV) signs, U+1A6E,
U+1A6F, U+1A70, U+1A71, U+1A72, need to be reordered to appear
*before* a consonant cluster (CC) which may include MEDIAL RA (w/MR):

                                             <CC> + <PDV>  => <PDV><CC>
                                   <CCw/MR> + <PDV>  =>  <PDV><CCw/MR>

That's about it!

There are other features of Tai Tham, such as subjoined consonants,
but I believe only the 2 features above require specialized shaper
code while everything else can be achieved using standard OpenType
features such as GSUB without resorting to specialized shaper code.

Note that if Tai Tham is implemented, then it will be simple to
implement other 'Tai' scripts like Tai Viet and New Tai Lue that have
very similar features.

Anyone interested in helping sort this out, please contact me.  Thanks!  - Ed