Not hooked up just yet.

Correspond to the make_mutable() ones.

One in particular is not a straight getter: hb_font_unset_funcs() is
special because of the specific needs of the lifecycle management of
the user_data object.

See email thread "Should MarkToBase attachment zero the mark advance?"
started by Jonathan Kew on 23 August 2010 for details.

No semantic change.

Got to add a suitable replacement.

Reported by Jonathan Kew.  Face table handling needs to be redone
anyway, but fix this for now.

Patch by Jonathan Kew.  Mozilla bug #465728.

Helps with avoiding many "extern C" declarations in source files.