Move the empty-buffer check from hb_shape_full() to hb_shape_plan_execute().

Reported by Simon Cozens.

Based on data from http://upstream-tracker.org/versions/harfbuzz.html
Resolves #103

Previously, when creating an object from inert inputs (eg:
"hb_font_create(hb_face_get_empty())") we returned the inert
empty object.  This is not helpful as there are legitimate
usecases to do that.

We now never return the inert object unless allocation failed.

Tests are revised to reflect.

The only effect is, if shaper_list was not NULL and no shaper was found,
we now don't insert anything into cache and return earlier.

Wasn't breaking out of loop, ouch!

http://www.mail-archive.com/debian-bugs-dist@lists.debian.org/msg1246370.html

Bug 77800 - cppcheck reports

Will fix by removing shape_plan->face completely.

Since our types are not associated with their methods, marking
constructors makes them inaccessible from bindings.  Undo for now.

Untested.

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.)

https://bugs.freedesktop.org/show_bug.cgi?id=53233

Should give us some performance boost.

Shapers have a chance to reject a font in face shaper_data creation.
No need to allow failing during planning.

Not optimized yet.  Eats babies.  And no shaper uses the shape_plan.

Add global shape_plan.  Unused so far.