This new API allows buffer implementations to know when a user is
actively accessing the buffer's underlying storage. This is
important for the upcoming client-backed wlr_buffer implementation.
This allows users to know the capabilities of the buffers that
will be allocated. The buffer capability is important to
know when negotiating buffer formats.
It allocates in local main memory via shm_open, and provides a FD
to allow sharing with other processes.
This is suitable for software rendering under the Wayland and X11
backends.