[HarfBuzz] Thai, revisited

[Jonathan Kew]

It turns out that some legacy Thai fonts provide OpenType substitution features to implement mark positioning, but (incorrectly) put those features/lookups under the 'latn' script tag instead of using 'thai' (or possibly 'DFLT'). See https://bugzilla.mozilla.org/show_bug.cgi?id=719366 for an example and more detailed description.

Although this is really a font bug, I suggest that we could improve the rendering of such fonts by looking for the 'latn' as a fallback if neither the requested script nor "default" is found in hb_ot_layout_table_choose_script. Suggested patch against harfbuzz master is attached.

This does _not_ affect the other kind of legacy Thai font, where custom code to support vendor-specific PUA codepoints would be needed. I'm not keen to go down that path; IMO, such fonts should be ruthlessly stamped out in favour of standards-based solutions. :)

JK

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