(mac_draw_string_common): Remove arg MODE. New arg

(mac_draw_string_common): Remove arg MODE.  New arg
BG_WIDTH.  All uses changed.  Draw background if BG_WIDTH is not zero.
(mac_draw_image_string, mac_draw_image_string_16): New arg BG_WIDTH.
[USE_CG_TEXT_DRAWING] (mac_draw_image_string_cg): Rename from
mac_draw_string_cg.  New arg BG_WIDTH.  All uses changed.  Draw
background if BG_WIDTH is not zero.  Use float constants as
divisors instead of double.  Use alloca instead of xmalloc/xfree.
(x_draw_glyph_string_background, x_draw_glyph_string_foreground)
[!MAC_OS8 || USE_ATSUI]: Background may be drawn using
mac_draw_image_string* functions.
(XLoadQueryFont) [MAC_OS8 && USE_ATSUI]: Don't adjust heights of
some fonts when srcCopy text transfer mode might be used.
(mac_begin_clip, mac_end_clip): Check if region is empty.
(mac_set_clip_rectangles): When resetting clip region, make it
empty instead of disposing of it.