If USE_FONT_BACKEND is defined, include "font.h".

If USE_FONT_BACKEND is defined, include "font.h".
Through out the file, use FONT_INFO_FROM_FACE instead of
FONT_INFO_FROM_ID, use get_per_char_metric instead of
rif->per_char_metric.
(handle_composition_prop) [USE_FONT_BACKEND]: If the composition
method is COMPOSITION_WITH_GLYPH_STRING, just set it->c to ' '.
(get_glyph_face_and_encoding, fill_composite_glyph_string)
(get_char_face_and_encoding, BUILD_COMPOSITE_GLYPH_STRING)
(x_produce_glyphs) [USE_FONT_BACKEND]: If enable_font_backend is
nonzero, use font-backend mechanism.
(get_per_char_metric): New function.