Replace XChar2b with unsigned in all font backends

Replace XChar2b with unsigned in all font backends

The xfont backend using XChar2b in its API doesn't mean that we should
use it everywhere else.

* src/dispextern.h (glyph_string):
* src/ftcrfont.c (ftcrfont_draw):
* src/ftxfont.c (ftxfont_draw):
* src/w32term.c (w32_draw_glyphless_glyph_string_foreground):
* src/xdisp.c (init_glyph_string, get_char_face_and_encoding)
(get_glyph_face_and_encoding, get_char_glyph_code)
(fill_gstring_glyph_string, fill_stretch_glyph_string)
(normal_char_ascent_descent, gui_get_glyph_overhangs)
(compute_overhangs_and_x, gui_produce_glyphs):
* src/xfont.c (xfont_get_pcm, xfont_chars_supported, xfont_open)
(xfont_encode_char, xfont_text_extents, xfont_draw)
* src/xftfont.c (xftfont_draw):
* src/xterm.c (x_compute_glyph_string_overhangs)
(x_draw_glyphless_glyph_string_foreground): Use unsigned over XChar2b.

* src/nsgui.h:
* src/w32gui.h:
* src/xterm.h: Remove XChar2b, STORE_XCHAR2B, XCHAR2B_BYTE1, and
XCHAR2B_BYTE2 typedefs and macros.

* src/font.h (font_driver):
(ftfont_text_extents)
* src/ftcrfont.c (ftcrfont_text_extents):
* src/ftfont.c (ftfont_text_extents):
* src/macfont.m (macfont_text_extents):
* src/nsfont.m (nsfont_text_extents):
* src/w32font.h (w32_font_text_extents):
* src/font.c (xfont_text_extents):
* src/xftfont.c (xftfont_text_extents): Make code parameter const.