Based on patch from Martin Hosken.  I believe it returns NULL if the
font doesn't have graphite tables, but have not tested.

Part of patch from Martin Hosken.

Patch from Martin Hosken.

Also remove shaper_options argument to hb_shape_full().  That was
unused and for "future".  Let it go.

More shaper API coming in preparation for plan/planned API.

Doesn't seem to be slower.

Still not sure about:

1) Case.  We pass lowercase for now.  Would be nice if graphite was
uppercase 3letter like OpenType,

2) Padding.  IMO, tag padding is always with spaces, but Martin was
talking about NUL bytes.

Can be -1 for NUL-terminated string.  This is useful for passing parts
of a larger string to a function without having to copy or modify the
string first.

Affected functions:

	hb_tag_t hb_tag_from_string()
	hb_direction_from_string()
	hb_language_from_string()
	hb_script_from_string()

I don't see how this function can be useful.

To be modified, a lot.