We used to rely on the order in which the
weston_compositor::destroy_signal callbacks happened, to not access
freed memory. Don't know when, but this broke at least with ivi-shell,
which caused crashes in random places on compositor shutdown.

Valgrind found the following:

 Invalid write of size 8
    at 0xC2EDC69: unbind_input_panel (input-panel-ivi.c:340)
    by 0x4E3B6BB: destroy_resource (wayland-server.c:537)
    by 0x4E3E085: for_each_helper.isra.0 (wayland-util.c:359)
    by 0x4E3E60D: wl_map_for_each (wayland-util.c:365)
    by 0x4E3BEC7: wl_client_destroy (wayland-server.c:675)
    by 0x4182F2: text_backend_notifier_destroy (text-backend.c:1047)
    by 0x4084FB: wl_signal_emit (wayland-server-core.h:264)
    by 0x4084FB: main (compositor.c:5465)
  Address 0x67ea360 is 208 bytes inside a block of size 232 free'd
    at 0x4C2A6BC: free (vg_replace_malloc.c:473)
    by 0x4084FB: wl_signal_emit (wayland-server-core.h:264)
    by 0x4084FB: main (compositor.c:5465)

 Invalid write of size 8
    at 0x4E3E0D7: wl_list_remove (wayland-util.c:57)
    by 0xC2EDEE9: destroy_input_panel_surface (input-panel-ivi.c:191)
    by 0x4E3B6BB: destroy_resource (wayland-server.c:537)
    by 0x4E3BC7B: wl_resource_destroy (wayland-server.c:550)
    by 0x40DB8B: wl_signal_emit (wayland-server-core.h:264)
    by 0x40DB8B: weston_surface_destroy (compositor.c:1883)
    by 0x40DB8B: weston_surface_destroy (compositor.c:1873)
    by 0x4E3B6BB: destroy_resource (wayland-server.c:537)
    by 0x4E3E085: for_each_helper.isra.0 (wayland-util.c:359)
    by 0x4E3E60D: wl_map_for_each (wayland-util.c:365)
    by 0x4E3BEC7: wl_client_destroy (wayland-server.c:675)
    by 0x4182F2: text_backend_notifier_destroy (text-backend.c:1047)
    by 0x4084FB: wl_signal_emit (wayland-server-core.h:264)
    by 0x4084FB: main (compositor.c:5465)
  Address 0x67ea370 is 224 bytes inside a block of size 232 free'd
    at 0x4C2A6BC: free (vg_replace_malloc.c:473)
    by 0x4084FB: wl_signal_emit (wayland-server-core.h:264)
    by 0x4084FB: main (compositor.c:5465)

 Invalid write of size 8
    at 0x4E3E0E7: wl_list_remove (wayland-util.c:58)
    by 0xC2EDEE9: destroy_input_panel_surface (input-panel-ivi.c:191)
    by 0x4E3B6BB: destroy_resource (wayland-server.c:537)
    by 0x4E3BC7B: wl_resource_destroy (wayland-server.c:550)
    by 0x40DB8B: wl_signal_emit (wayland-server-core.h:264)
    by 0x40DB8B: weston_surface_destroy (compositor.c:1883)
    by 0x40DB8B: weston_surface_destroy (compositor.c:1873)
    by 0x4E3B6BB: destroy_resource (wayland-server.c:537)
    by 0x4E3E085: for_each_helper.isra.0 (wayland-util.c:359)
    by 0x4E3E60D: wl_map_for_each (wayland-util.c:365)
    by 0x4E3BEC7: wl_client_destroy (wayland-server.c:675)
    by 0x4182F2: text_backend_notifier_destroy (text-backend.c:1047)
    by 0x4084FB: wl_signal_emit (wayland-server-core.h:264)
    by 0x4084FB: main (compositor.c:5465)
  Address 0x67ea368 is 216 bytes inside a block of size 232 free'd
    at 0x4C2A6BC: free (vg_replace_malloc.c:473)
    by 0x4084FB: wl_signal_emit (wayland-server-core.h:264)
    by 0x4084FB: main (compositor.c:5465)

Looking at the first of these, unbind_input_panel() gets called when the
text-backend destroys its helper client which has bound to input_panel
interface. This happens after the shell's destroy_signal callback has
been called, so the shell has already been freed.

The other two errors come from
  wl_list_remove(&input_panel_surface->link);
which has gone stale when the shell was destroyed
(shell->input_panel.surfaces list).

Rather than creating even more destroy listeners and hooking them up in
spaghetti, modify text-backend to not hook up to the compositor destroy
signal. Instead, make it the text_backend_init() callers' responsibility
to also call text_backend_destroy() appropriately, before the shell goes
away.

This fixed all the above Valgrind errors, and avoid a crash with
ivi-shell when exiting Weston.

Also using desktop-shell exhibited similar Valgrind errors which are
fixed by this patch, but those didn't happen to cause any crashes AFAIK.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-By: Derek Foreman <derekf@osg.samsung.com>