[truetype] Handle inner/outer index values 0xFFFF/0xFFFF.

This was introduced in OpenType 1.8.4.

* src/truetype/ttgxvar.c (ft_var_load_delta_set_index_mapping,
tt_hvadvance_adjust, ft_var_load_mvar, tt_apply_mvar): Handle special
inner/outer index values.
(ft_var_load_item_variation_store): Add test.

Fixes #1154.
1 file changed