(x_append_glyph): Setup members of struct glyph properly

(x_append_glyph): Setup members of struct glyph properly
for composition.
(x_append_composite_glyph): New function.
(VCENTER_BASELINE_OFFSET): New macro.
(x_produce_glyphs): If it->what == IT_COMPOSITION, setup members
of struct it for the composition.  Cache pixel offsets in the
struct composition.  Delete codes for a composite character.
Handle Vignore_relative_composition in composition code.
(struct glyph_string): Delete member cmpcharp, add new member cmp.
(x_set_cursor_gc): Check s->cmp, not s->cmpcharp.
(x_compute_glyph_string_overhangs): Likewise.
(x_get_glyph_overhangs): Delete codes for a composite character.
(x_right_overwritten): Check s->cmp, not s->cmpcharp.
(x_draw_glyph_string_background): Likewise.  Delete codes for
checking s->gidx for a composition.
(x_draw_glyph_string_foreground): Delete code for a composite
character.
(x_draw_composite_glyph_string_foreground): New function.
(x_draw_glyph_string_box): Check s->cmp, not s->cmpcharp.
(x_draw_glyph_string): Handle the case of COMPOSITE_GLYPH.
(struct work): Deleted.
(x_fill_composite_glyph_string): Argument changed.  Mostly
rewritten for that.
(x_fill_glyph_string): Don't check CHARSET_COMPOSITION.
(BUILD_CHAR_GLYPH_STRINGS): Don't handle composition here.
(BUILD_COMPOSITE_GLYPH_STRING): New macro.
(BUILD_GLYPH_STRINGS): For composition, call
BUILD_COMPOSITE_GLYPH_STRING.
(x_new_font): Initialize f->output_data.x->baseline_offset, not
f->output_data.x->font_baseline.