For two reasons:

1. User can always call hb_buffer_pre_allocate() themselves, and

2. Now we do a pre_alloc in add_utfX anyway, so the total number of
reallocs is limited to a small number (~3) anyway.  This just makes the
API cleaner.

Using graphite2's comparerenderer suggests that this makes hb-ft 15
times faster.  No caching layer needed anymore.

We do a conservative estimate of the number of characters, but still,
this limits the number of buffer reallocs to a small constant.

According to Peter Constable this is indeed what Uniscribe has been
doing for years.

Mozilla Bug 667166 - wrong shape of letter when it comes at the end of
word in the arabic version of Firefox 5.0

If the specified shapers fail, hb-shape will fail immediately

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'.