(GC_CLIP_REGION): Remove macro.

(GC_CLIP_REGION): Remove macro.
(mac_begin_clip, mac_end_clip): Take arg GC instead of REGION.
All uses changed.  Don't do clipping if n_clip_rects is zero.
(mac_set_clip_rectangles): Use xassert instead of abort.  Set
n_clip_rects.  Don't make clip_region empty when number of
clipping rectangles is zero.
(mac_reset_clip_rectangles): Set n_clip_rects directly instead of
calling mac_set_clip_rectangles.
(x_set_toolkit_scroll_bar_thumb): Temporarily hide scroll bar to
avoid multiple redraws.