[HarfBuzz] fallback kerning

[Jonathan Kew]

Hi Behdad,

We're seeing some widespread fonts (such as Verdana and Trebuchet) where there is a GPOS table that does *not* include a 'kern' feature, and an old-style TrueType 'kern' table is present. To support fonts like this, I think we need to modify the positioning logic so that the use of the fallback positioning code is not dependent simply on the presence of a GPOS table, but on whether it actually includes a 'kern' feature.

Suggested patch is attached. This checks at feature-compilation time whether a 'kern' feature is present (or if it has been explicitly disabled), and returns a boolean to indicate whether kerning was "handled" at this level; if not, the fallback path will be used.

Jonathan

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