[HarfBuzz] unbreaking mixed-up khmer fonts

[Jonathan Kew]

Hi Behdad,

Hope you've had a good weekend - thanks for all the good work last week.

One thing has come up: it seems we've broken things for some widely-used 
Khmer fonts. E.g. the font "hanuman.ttf" used on http://khmer.rfa.org/. 
This font has a 'liga' feature, but nevertheless relies on going through 
the indic shaper; it duplicates many of its lookups from 'abvs', etc., 
into 'liga' and 'clig', but it does not use these features to do the 
pre-base vowel or subjoined consonants.

The result is that it fails when shaped using the generic shaper, but 
works ok if we use the indic shaper with 'liga' disabled (because the 
features it puts in liga are just references to the lookups that are 
also in abvs etc, so ignoring liga is fine).

And so I think we need to revert the Khmer part of 
981748cb2e9b48b77177b19ec1f972cab7afda89 (but keep the Myanmar part), 
and rely only on 6b389ddc3623d042ded4731f4d62dc354002fdd0 to deal with 
the pre-base duplication that we were seeing with fonts like 
Kh-Battambang. Remind me if there's something else that would break?

JK