[interpolatable] WIP use uharfbuzz if available

For faster drawing. However, I'm hitting a lot of
node-count-differs errors.  I have not tracked the
cause down yet.

I thought this line might be:

https://github.com/harfbuzz/harfbuzz/blob/df635ab78ae1d397396e3a0fe688b8a4674b2e8a/src/hb-draw.hh#L148

But disabling it in uharfbuzz didn't help.