[HarfBuzz] OTF font with CALT alternative turned on by default

[Liam]

Hello again,

     I am having an issue with a certain font and how to handle it with 
the shaper. Sadly I can not post this font but hopefully I can explain 
this well enough to get a correct answer.

This font has turned on by default the CALT table, and it also behaves 
differently compared to most open type fonts that I am testing with.

I tested this font using adobe illustrator and by default it does grab 
the 'f' glyph from the calt table, but I can turn it off. Is there a way 
to have the hb_shaper to not use alternate glyph substitutions?

My other option at the moment is to check for a null value for the 
features list and just use FreeType for the lookup.

Thanks