Include fontset.h after dispextern.h.

Include fontset.h after dispextern.h.
(x_per_char_metric): Don't try FONT->default_char.  Even if
pcm->width is zero, glyph bits may exist.
(x_encode_char): Always initialize char2b->byte1.
(x_get_char_face_and_encoding): Call FACE_FOR_CHAR to get face_id.
(x_get_glyph_face_and_encoding): New arg two_byte_p.  Caller
changed.
(x_append_glyph): Set glyph->glyph_not_available_p.
(x_produce_glyphs): Set it->glyph_not_available_p.  Don't set
it->charset.  Handle the case that per char metric is not
available.  If it->multibyte_p is zero and it->c is a multibyte
character, convert it to a unibyte character.
(struct glyph_string): Delete member `charset'.
(x_set_mouse_face_gc): Call FACE_FOR_CHAR to get face_id.  Handle
the case that per char metric is not available correctly.
(x_fill_glyph_string): Handle the case that the specific glyph is
not available correctly.
(BUILD_CHAR_GLYPH_STRINGS): Don't set s->charset.
(BUILD_COMPOSITE_GLYPH_STRING): Likewise.
(x_new_font): Call FS_LOAD_FONT, not fs_load_font.
(x_new_fontset): Call fontset_ascii to get ASCII font name of the
fontset.  Don't call FS_LOAD_FONT.