Throughout the file, delete all USE_FONT_BACKEND

Throughout the file, delete all USE_FONT_BACKEND
conditionals.  Don't check enable_font_backend.  Delete all codes
used only when USE_FONT_BACKEND is not defined.
(handle_auto_composed_prop): Do nothing if it->f is not on a
window system.  Check how many following characters can be
displayed by the same font.
(calc_pixel_width_or_height): Type of the 4th arg is changed to
'struct font *'.
(get_char_face_and_encoding): Assign the whole encoding task to
the `encode-char' method of a font driver.
(fill_composite_glyph_string): Adjusted for the change of `struct
face' and `struct glyph_string'.
(fill_glyph_string): Likewise.
(get_per_char_metric): Arguments changed.
(x_get_glyph_overhangs): Adjusted for the change of `struct face'
and `struct glyph_string'.
(produce_stretch_glyph, calc_line_height_property)
(x_produce_glyphs): Likewise.