* dispextern.h (struct glyph_string): New members clip_head and

* dispextern.h (struct glyph_string): New members clip_head and
	clip_tail.
* xdisp.c (get_glyph_string_clip_rect): Restrict horizontal clip
	region to the area between clip_head and clip_tail.
	(draw_glyphs): Record the area that need to be actually redrawn to
	the new variables clip_head and clip_tail when there are
	overhangs.  Set values of these variables to the corresponding
	members in struct glyph_string.  Refine x coordinates for
	notice_overwritten_cursor using clip_head and clip_tail.
* macgui.h (STORE_XCHARSETSTRUCT): New macro.
* macterm.c (mac_compute_glyph_string_overhangs): Implement with
	QDTextBounds.
	(x_draw_glyph_string): Don't fill the background of the successor
	of a glyph with a right overhang if the successor will draw a
	cursor.
	(XLoadQueryFont): Obtain font metrics using QDTextBounds.
	(x_redisplay_interface): Add entry for
	compute_glyph_string_overhangs.