wayland: Rewrite wl-shm implementation

The main motivation for the rewrite is to properly port ShmClientBuffer
to our GraphicsBuffer abstractions. As is, libwayland implementation is
not suitable for our needs. We may need ShmClientBuffer to be alive when
its corresponding wl_shm_buffer resource is destroyed, for example to
play a window closing animation. With the existing api, we need to fight
libwayland. Besides that, libwayland doesn't provide a way to get
underlying pool file descriptor, which is needed to fill ShmAttributes.