Pango2: Adopted the optical-margins example for harfbuzz >= 5.3.0

Harfbuzz 5.3.0 (specifically 71c6cba) renamed its hb_ot_layout_get_optical_bound function to hb_ot_layout_lookup_get_optical_bound. This patch simply replaces the call to the old function with a call to the renamed one, that pango2 can be built with newer harfbuzz versions.