shm: Defer wl_shm_pool_resize if a pool has external references

If a compositor is rendering in one thread while dispatching wayland
events in another, a wl_shm_pool_resize() could change the memory
mappings it's rendering from and cause a crash.

Now we defer wl_shm_pool_resize() if the compositor has references on a
pool, and perform the actual resize when it drops those references.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>