Remove hb_ft_get_font_funcs() as it cannot be used by the user anyway.

Add hb_ft_font_set_funcs().  Which will make the font internally use
FreeType.  That is, no need for the font to have created using the
hb-ft API.  Just create using hb_face_create()/hb_font_create() and
then call this on the font (after having set font scale).  This
internally creates an FT_Face and attached to the font.

We need this to set data on objects safely without worrying that some
other thread unsets it by setting it at the same time.

Since we already switched to accepting -1 as 'zero-terminated'.

hb_face_get_blob() renamed to hb_face_reference_blob(), returns a
reference now.

hb_face_[sg]et_index() added.

hb_face_set_upem() added.

It's kinda working finally!

A -1 text_length means: zero-terminated string.
A -1 item_length means: to the end of string.

Oops!

Not tested yet.

automake test runner understands this.