[HarfBuzz] How to get hb_face_t and hb_font_t without Freetype?

[Eli Zaretskii]

> From: Jonathan Kew <jfkthame at gmail.com>
> Date: Sat, 11 May 2019 11:08:42 +0100
> 
> You can use hb_face_create_for_tables, passing it a function that can 
> retrieve font tables (as hb_blobs) when requested by harfbuzz.
> 
> This is what Firefox does, to use harfbuzz with Windows or MacOS font 
> APIs; a starting point to explore the Firefox code would be [1], where 
> we call hb_face_create_for_tables and pass it HBGetTable as the 
> reference_table_func. This calls down to the GetFontTable() method, 
> which has separate implementations for the various platforms.

Thanks, this gets me a notch forward, but I'm afraid there's still a
lot of fog.  Specifically, what does HarfBuzz expect from the
hb_blob's it retrieves this way, and where and how does it use those
blobs?