Disable automatic segment properties guessing

Before, if one called hb_shape() without setting script, language, and
direction on the buffer, hb_shape() was calling
hb_buffer_guess_segment_properties() on the user's behalf to guess
these.

This is very dangerous, since any serious user of HarfBuzz must set
these properly (specially important is direction).  So now, we don't
guess properties by default.  People not setting direction will get
an abort() now.  If the old behavior is desired (fragile, good for
simple testing only), users can call
hb_buffer_guess_segment_properties() on the buffer just before calling
hb_shape().
4 files changed