wayland/dma-buf: Add meta_wayland_dma_buf_fds_for_wayland_buffer

And call it from meta_wayland_buffer_realize. This makes dma-buf fds
available for EGL image type buffers as well.

v2:
* Move buffer->dma_buf.dma_buf assignment value to next line.
  (Jonas Ådahl)

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1880>

src/wayland/meta-wayland-buffer.c

@@ -184,6 +184,8 @@ meta_wayland_buffer_realize (MetaWaylandBuffer *buffer)
                                          NULL))
         {
           buffer->type = META_WAYLAND_BUFFER_TYPE_EGL_IMAGE;
+          buffer->dma_buf.dma_buf =
+            meta_wayland_dma_buf_fds_for_wayland_buffer (buffer);
           return TRUE;
         }
     }