Fix display of composed text with :box face attribute

Fix display of composed text with :box face attribute

* src/xdisp.c (get_next_display_element): For a composition on a
display or overlay string, set the end_of_box_run_p flag if the
string ends at the last character included in the composition.
(fill_gstring_glyph_string): Fix the way the width of a gstring
glyph string is calculated: use the values calculated in
gui_produce_glyphs, since the latter adjusts the width due to the
face's ':box' attribute.
* src/xterm.c (x_draw_glyph_string_box):
* src/w32term.c (w32_draw_glyph_string_box):
* src/nsterm.m (ns_dumpglyphs_box_or_relief): Support automatic
compositions, which have the right_box_line_p flag set on the last
glyph produced from the composition.  (Bug#40687)

* src/w32term.c (w32_compute_glyph_string_overhangs): Update to be
consistent with xterm.c in its support of automatic composition
glyph strings.
* src/dispextern.h (enum glyph_type): More accurate commentary.
* src/.gdbinit (pgx): Display slice.img members only for image
glyphs.