buffer: re-use wlr_shm_client_buffer

The first time wlr_buffer_from_resource is called with a wl_buffer resource that originates from wl_shm, create a new wlr_shm_client_buffer as usual. If wlr_buffer_from_resource is called multiple times, re-use the existing wlr_shm_client_buffer.

This commit changes how the wlr_shm_client_buffer lifetime is managed: previously it was destroyed as soon as the wlr_buffer was released. With this commit it's destroyed when the wl_buffer resource is.

Apart from de-duplicating wlr_shm_client_buffer creations, this allows to easily track when a wlr_shm_client_buffer is re-used. This is useful for the renderer and the backends, e.g. the Pixman renderer can keep using the same Pixman image if the buffer is re-used. In the future, this will also allow to re-use resources in the Wayland and X11 backends (remote wl_buffer objects for Wayland, pixmaps for X11).