libweston: Make weston_pointer destruction safe

libweston: Make weston_pointer destruction safe

Properly clean up all sub-objects (e.g., weston_pointer_client objects)
when a weston_pointer object is destroyed. The clean-up ensures that the
server is able to safely handle client requests to any associated
pointer resources, which, as a consenquence of a weston_pointer
destruction, have now become inert.

The clean-up involves, among other things, unsetting the destroyed
weston_pointer object from the user data of pointer resources, and
handling this NULL user data case where required. Note that in many
sites affected by this change the existing code already properly handles
NULL weston_pointer (e.g. in init_pointer_constraint), so there is no
need for additional updates there.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>