relative_pointer: handle inert pointer objects

Since 5e0ef70cc085 ("seat: Create inert objects for missing capabilities")
wlroots can create inert seat objects when the capability is currently missing
for the client but it had the capablity before. The client hoever will happily
handover the wl_pointer resource to the relative_pointer implementation,
creating a NULL pointer dereference when trying to access the seat_client which
is set to NULL for inert objects.

Since the protocol does not contain an error for such requests, we hand out an
relative_pointer handle with the seat set to NULL.
We also need to check whether there is an associated seat in
send_relative_motion and need to tweak the destroy notifier in case no seat is
available.

This way we can hand out a valid relative_pointer resource and don't crash the
compositor when trying to access an inert seat pointer resource in
relative_pointer.

Relevant WAYLAND_DEBUG=1 when testing a client and switching VT every second:
[2619872.442] wl_seat@30.capabilities(3)
[2619872.460]  -> wl_seat@30.get_pointer(new id wl_pointer@36)
[2619872.484] wl_data_device@25.selection(nil)
[2619872.504] zwp_primary_selection_device_v1@26.selection(nil)
[2619874.995] wl_seat@12.capabilities(3)
[2619875.035]  -> wl_compositor@5.create_surface(new id wl_surface@37)
[2619875.088]  -> wl_seat@12.get_pointer(new id wl_pointer@29)
[2619875.105]  -> zwp_relative_pointer_manager_v1@8.get_relative_pointer(new id zwp_relative_pointer_v1@27, wl_pointer@29)
[2619875.127]  -> wl_compositor@5.create_surface(new id wl_surface@35)
[2619875.139]  -> wl_seat@12.get_pointer(new id wl_pointer@43)
[2619981.180] wl_seat@12.capabilities(2)
[2619981.214]  -> zwp_relative_pointer_v1@27.destroy()
[2619981.226]  -> wl_pointer@29.release()
[2619981.236]  -> wl_surface@37.destroy()
[2619981.247]  -> wl_pointer@43.release()
[2619981.254]  -> wl_surface@35.destroy()
[2619981.262] wl_seat@12.capabilities(0)
[2619981.285]  -> wl_keyboard@33.release()
[2619987.316] wl_seat@30.capabilities(2)
[2619987.336]  -> wl_pointer@36.release()
[2619987.363] wl_seat@30.capabilities(0)
[2619987.371]  -> wl_keyboard@34.release()
[2621932.880] wl_display@1.delete_id(41)
[2621932.903] wl_display@1.delete_id(40)
[2621932.910] wl_display@1.delete_id(27)
[2621932.917] wl_display@1.delete_id(29)
[2621932.924] wl_display@1.delete_id(37)
[2621932.930] wl_display@1.delete_id(43)
[2621932.944] wl_display@1.delete_id(35)
[2621932.950] wl_display@1.delete_id(33)
[2621932.959] wl_seat@12.capabilities(2)
[2621932.976]  -> wl_seat@12.get_keyboard(new id wl_keyboard@33)
[2621936.875] wl_seat@12.capabilities(3)
[2621936.893]  -> wl_compositor@5.create_surface(new id wl_surface@35)
[2621936.931]  -> wl_seat@12.get_pointer(new id wl_pointer@43)
[2621936.945]  -> zwp_relative_pointer_manager_v1@8.get_relative_pointer(new id zwp_relative_pointer_v1@37, wl_pointer@43)
[2621936.965]  -> wl_compositor@5.create_surface(new id wl_surface@29)
[2621936.987]  -> wl_seat@12.get_pointer(new id wl_pointer@27)
[2621942.796] wl_data_device@25.selection(nil)
[2621942.817] zwp_primary_selection_device_v1@26.selection(nil)
[2621942.823] wl_seat@30.capabilities(2)

0 files changed, 0 insertions, 0 deletions