Use a basic linked list to store the currently active mappings.
Note that we don't actually need to implement a full lock-free
atomic linked list here. The signal handler will never write to
the list, it will only read it. Only the main thread will write.
We need to always expose a consistent view of the list to the
signal handler (the main thread might be interrupted at any point
by the signal handler).
This is a re-implementation of wl_shm. The motivations for using
this over the one shipped in libwayland are:
- Properly handle SIGBUS when accessing a wl_buffer's underlying
data after the wl_buffer protocol object has been destroyed.
With the current code, we just crash if the client does that
and then shrinks the backing file.
- No need to fight the wl_shm_buffer API anymore. This was awkward
because we weren't notified when clients created a wl_shm buffer,
and this doesn't play well with our wlr_buffer abstraction.
- Access to the underlying FD. This makes it possible to forward
the wl_shm buffer to a parent compositor with the Wayland/X11
backends.
- Better stride checks. We can use our format table to ensure that
the stride is consistent with the bpp and width.
