Return type of hb_subset_plan_old_to_new_glyph_mapping() et al

[behdad]

They currently return const hb_map_t *. That is understandable. The reason we don't return const objects in the API though is that hb_map_reference et al take non-const. They can be made to take const, but their return value has to choose between const or non-const. So historically they take non-const and return non-const.

Right now if one wants to reference the return of hb_subset_plan_old_to_new_glyph_mapping and family they need a cast, which is undesirable. I suggest we change them to drop the const. After all, we don't care if user modifies these.

[behdad]

cc @garretrieger