(handle_composition_prop): Set it->c to the fist

(handle_composition_prop): Set it->c to the fist
characte of the composed region.
(fill_composite_glyph_string): Set base_face->font_info to
s->font_info.  Get a face for ascii from base_face->ascii_face.
(BUILD_COMPOSITE_GLYPH_STRING): Call fill_composite_glyph_string
with a face already decided.
(x_produce_glyphs): Be sure to set it->ascent and it->descent to
non-negative.
(x_produce_glyphs): If the composition method is
..._WITH_GLYPH_STRING, call font_prepare_composition
unconditionally.