This caused crashes when using highlight_trailing_whitespace on X.
The old frame pointer was resurrected when the glyph was reused, and
pointed to a dead object.

* src/xdisp.c (append_glyph):
(append_composite_glyph):
(produce_image_glyph):
(produce_xwidget_glyph):
(append_stretch_glyph):
(append_glyphless_glyph): Initialize glyph->frame.

* src/ftfont.c (syms_of_ftfont): Move code inside HAVE_HARFBUZZ section.

* src/igc.c (root_create_coding_system): New helper.
(igc_alloc_coding_system): Use it.
(make_igc): Create a root for safe_terminal_coding.

* src/term.c (tty_menu_show): Always use SAFE_ALLOCA for menu items.

* src/ftfont.c (syms_of_ftfont): Staticpro fthbfont_driver.type.
* src/ftfont.c (syms_of_ftcrfont): Staticpro ftcrhbfont_driver.type.

That's simpler; at the cost of pinning more objects.

* src/igc.c (scan_coding_system): At RankAMBIG, we can fix interior
pointers so the code can be simpler.
(igc_alloc_coding_system): Create an ambiguous root.

As struct coding_system is now scanned as exact root, fixing coding
systems in fix_terminal should no longer be needed.

* src/igc.c (fix_coding): Deleted.
(fix_terminal): Don't scan coding systems.

* src/igc.h (igc_alloc_coding_system): New prototype.
* src/igc.c (igc_alloc_coding_system, scan_coding_system): New.
* src/terminal.c (create_terminal): Use igc_alloc_coding_system.
* src/process.c (Fprocess_send_eof, setup_process_coding_systems): Use
igc_alloc_coding_system.