Introduce wlr_shm

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.

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