[HarfBuzz] get/set_user_data()

[Behdad Esfahbod]

So, are people actually using the get/set_user_data() methods on HB objects?
I'm thinking about removing them at least for hb_blob_t, as part of
streamlining blobs and making them more lightweight.  But if no one uses the
get/set_user_data(), I may as well just remove them all.

Initially I added these because (based on cairo experience) I was under the
impression that they are extremely useful for language bindings.  However,
they are only if one is willing to write manual bindings, which is something I
prefer to avoid.

So, if no existing user of HarfBuzz uses them, I think it's a good indication
that they were overkill design artifacts and I'll remove before 1.0.

Thanks,
-- 
behdad
http://behdad.org/