Tamil MN on 10.13.6 is broken

[ebraminio]

Unlike its 10.12.6 version which was fixed #1348, its 10.13.6 version is broken and it hasn't empty language list on gsub in order to be detected so it is broken.

[behdad]

Ugh. This one (Tamil MN on 10.13.6) has non-empty GSUB. So, back to needing to prefer morx/kerx on fonts that have GSUB/GPOS...

[behdad]

cc @drott

[ebraminio]

Obviously #1391 is the ultimate answer but maybe always favouring AAT over OT for Indic scripts as a workaround?

[behdad]

Given that the Khmer issue is resolved I'm back to thinking maybe we should just add exception to blacklist GSUB for MN fonts (ie. OS/2 VendorID 'MUTF').

[drott]

SGTM, those seem to be the only other odd ones that we have encountered so far. We should perhaps also report it to Apple at the same time.