Create shm buffer directly instead of using RawPool

Currently needed if we want an fd to pass to iced-sctk for subsurfaces.
Rather than a wl_buffer.

It would be good if this logic and the dmabuf type could be shared in
some crate.

Cargo.lock

@@ -938,6 +938,7 @@ dependencies = [
  "memmap2 0.9.4",
  "once_cell",
  "rust-embed",
+ "rustix 0.38.30",
  "tokio",
  "wayland-protocols 0.31.0",
  "zbus",