(mac_per_char_metric): Add prototype.

(mac_per_char_metric): Add prototype.
(x_per_char_metric) [USE_CG_TEXT_DRAWING]: Remove prototype.
(mac_query_char_extents): New function.
(x_per_char_metric): Use it.
(XLoadQueryFont): Likewise.  Consolidate min/max_bounds calculations.
[USE_CG_TEXT_DRAWING] (mac_draw_string_cg): Use
mac_per_char_metric instead of x_per_char_metric.
(mac_text_extents_16): New function.
(mac_compute_glyph_string_overhangs): Use it.
(mac_unload_font): Free member `bounds' in struct MacFontStruct.