(fringe_faces): Change to Lisp_Object pointer.

(fringe_faces): Change to Lisp_Object pointer.
(draw_fringe_bitmap_1): Lookup user defined fringe faces here.
(destroy_fringe_bitmap): Set fringe_faces element to nil.
(Fdefine_fringe_bitmap, init_fringe): Change allocation of
fringe_faces array and init elements to nil.
(Fset_fringe_bitmap_face): Set fringe_faces to face name instead of
non-persistent face id.
(mark_fringe_data): New function for GC.