[HarfBuzz] Thai below-base normalization

[Martin Hosken]

Dear All,

> It's not clear to me, then, why uniscribe treats this in the way it 
> does. (Perhaps there was no good reason, and it was merely an arbitrary 
> choice of ordering in the absence of any clear requirement?)

In my data, I have no examples of a U+0E3A occurring after/below U+0E38/9. So I agree with this patch. Note that U+0E3A does occur following upper vowels (U+0E34-7).

Yours,
Martin