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.

types/meson.build

@@ -71,6 +71,7 @@ wlr_files += files(
 	'wlr_screencopy_v1.c',
 	'wlr_server_decoration.c',
 	'wlr_session_lock_v1.c',
+	'wlr_shm.c',
 	'wlr_single_pixel_buffer_v1.c',
 	'wlr_subcompositor.c',
 	'wlr_switch.c',