(x_draw_phys_cursor_glyph): Redraw overlaps.

(x_draw_phys_cursor_glyph): Redraw overlaps.
(x_fix_overlapping_area): New.
(x_redisplay_interface): Add x_fix_overlapping_area.
(x_draw_glyphs): Add parameter overlaps_p.
(struct glyph_string): Add member for_overlaps_p.
(x_get_glyph_string_clip_rect): If glyph string draws foreground
of overlapping rows, clip to window bottom.
(x_fill_glyph_string): Add parameter overlaps_p.
(x_fill_composite_glyph_string): Ditto.
(BUILD_GLYPH_STRINGS): Ditto.
(BUILD_CHAR_GLYPH_STRINGS): Ditto.
(x_draw_glyph_string): Don't draw anything but the foreground
if glyph string draws row overlaps.
(x_append_glyph): Set glyph flag overlaps_vertically_p.
(x_produce_image_glyph): Compute iterator's physical
ascent and descent.
(x_produce_stretch_glyph): Ditto.
(x_produce_glyphs): Ditto.