(struct glyph): Reduce face_id member from 22 to

(struct glyph): Reduce face_id member from 22 to
21 bits (this reduces number of faces from 4M to 2M).
Replace W32 specific w32_font_type member (2 bits) by generic
font_type member (3 bits) for portability.
(FONT_TYPE_UNKNOWN): New define, default for font_type member.
(enum draw_glyphs_face): Define here.
(struct glyph_string): Define here.  Merge W32 and X versions.
(struct redisplay_interface): New members per_char_metric,
encode_char, compute_glyph_string_overhangs, draw_glyph_string.
(VCENTER_BASELINE_OFFSET): Define here.
(dump_glyph_string, x_get_glyph_overhangs, x_produce_glyphs)
(x_draw_glyphs, notice_overwritten_cursor): Declare prototypes here.