add asset loading indicator (and maybe streamed loading)

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https://api.github.com/mosure/bevy_gaussian_splatting/blob/51bdd5d4efa484d388a0fdbe1d9684c06746c418/src/render/mod.rs#L784

                }),
            },
        ],
    ));

    for (entity, cloud_handle) in gaussian_clouds.iter() {
        // TODO: add asset loading indicator (and maybe streamed loading)
        if Some(LoadState::Loading) == asset_server.get_load_state(cloud_handle) {
            continue;
        }

        if gaussian_cloud_res.get(cloud_handle).is_none() {
            continue;
        }