wayland/surface: Hold shm-buffers of subsurfaces

We do not hold shm-buffer resources as we have an internal copy. For subsurfaces this is problematic as they may switch from async to sync - if the resource is gone and we now want to process damage (thus copy parts of the buffer that we potentially did not copy already) we hit an assert.

Hold a reference to the resource until it's replaced instead.

Stops the gstreamer-wayland-demo to spam meta_wayland_buffer_process_damage: assertion 'buffer->resource' failed on move/resize.