https://github.com/behdad/harfbuzz/issues/360

Sample crash:

0  _hb_graphite2_shaper_face_data_destroy (data=0xffffffffffffffff)
    at ../../src/hb-graphite2.cc:129
1  0x00007ffff4271d7d in hb_graphite2_shaper_face_data_ensure (
    face=<optimized out>) at ../../src/hb-shaper-list.hh:35
2  hb_shape_plan_plan (shaper_list=<optimized out>, num_user_features=0,
    user_features=0x0, shape_plan=0xf7b490) at ../../src/hb-shaper-list.hh:35
3  hb_shape_plan_create (face=<optimized out>, props=<optimized out>,
    user_features=0x0, num_user_features=0, shaper_list=<optimized out>)
    at ../../src/hb-shape-plan.cc:108
4  0x00007ffff4272c93 in hb_shape_plan_create_cached (face=0x10cf2b0,
    props=0x11980d8, user_features=0x0, num_user_features=<optimized out>,
    shaper_list=0x0) at ../../src/hb-shape-plan.cc:283

API additions:

	hb_segment_properties_t
	HB_SEGMENT_PROPERTIES_DEFAULT
	hb_segment_properties_equal()
	hb_segment_properties_hash()

	hb_buffer_set_segment_properties()
	hb_buffer_get_segment_properties()

	hb_ot_layout_glyph_class_t

	hb_shape_plan_t
	hb_shape_plan_create()
	hb_shape_plan_create_cached()
	hb_shape_plan_get_empty()
	hb_shape_plan_reference()
	hb_shape_plan_destroy()
	hb_shape_plan_set_user_data()
	hb_shape_plan_get_user_data()
	hb_shape_plan_execute()

	hb_ot_shape_plan_collect_lookups()

API changes:

	Rename hb_ot_layout_feature_get_lookup_indexes() to
	hb_ot_layout_feature_get_lookups().

New header file:

	hb-shape-plan.h

And a bunch of prototyped but not implemented stuff.  Coming soon.
(Tests fail because of the prototypes right now.)

Add global shape_plan.  Unused so far.