Make struct font_drivers read-only

Make struct font_drivers read-only

This simplifies the code a bit, and makes the structs more
shareable and less likely to become corrupt.
* src/alloc.c (cleanup_vector):
* src/font.c (valid_font_driver, font_prepare_cache)
(font_finish_cache, font_get_cache, font_clear_cache)
(register_font_driver, font_update_drivers):
* src/font.h (struct font, struct font_driver_list)
(valid_font_driver):
struct font_drivers are now const.
* src/font.c, src/ftcrfont.c, src/ftfont.c, src/nsfont.m, src/xfont.c:
Omit no-longer-necessary decls.
* src/ftcrfont.c (syms_of_ftcrfont):
* src/ftxfont.c (syms_of_ftxfont):
* src/xftfont.c (syms_of_xftfont):
Omit no-longer-necessary initialization code.
* src/ftcrfont.c (ftcrfont_driver):
* src/ftfont.c (ftfont_driver):
* src/ftxfont.c (ftxfont_driver):
* src/macfont.m (macfont_driver):
* src/nsfont.m (nsfont_driver):
* src/xfont.c (xfont_driver):
* src/xftfont.c (xftfont_driver):
Use C99-style initializer for ease of maintenance, and make it const.
* src/ftcrfont.c, src/ftxfont.c, src/xftfont.c:
Refer to functions like ftfont_text_extents directly.
* src/ftfont.c (ftfont_get_cache, ftfont_list, ftfont_list_family)
(ftfont_has_char, ftfont_encode_char, ftfont_text_extents)
(ftfont_get_bitmap, ftfont_anchor_point, ftfont_otf_capability)
(ftfont_variation_glyphs, ftfont_filter_properties)
(ftfont_combining_capability):
* src/xfont.c (xfont_get_cache):
Now extern, so that other modules’ struct font_drivers can use
them directly.
* src/macfont.m (macfont_descriptor_entity):
* src/nsfont.m (nsfont_open):
Use constant directly; this is clearer.