怎么获取osgEarth::Layer的osgEarth::FeatureSource

要获取 `osgEarth::Layer` 的 `osgEarth::FeatureSource`，可以使用以下方法：

osgEarth::FeatureSource* getFeatureSource(osgEarth::Layer* layer)
{
    osgEarth::Features::FeatureModelLayer* featureModelLayer = dynamic_cast<osgEarth::Features::FeatureModelLayer*>(layer);
    if (featureModelLayer)
    {
        return featureModelLayer->getFeatureSource();
    }

    osgEarth::Features::FeatureSourceLayer* featureSourceLayer = dynamic_cast<osgEarth::Features::FeatureSourceLayer*>(layer);
    if (featureSourceLayer)
    {
        return featureSourceLayer->getFeatureSource();
    }

    return NULL;
}

该方法首先尝试将 `osgEarth::Layer` 转换为 `osgEarth::Features::FeatureModelLayer`，如果转换成功，则返回 `FeatureModelLayer` 的 `FeatureSource`。如果转换失败，则尝试将 `osgEarth::Layer` 转换为 `osgEarth::Features::FeatureSourceLayer`，如果转换成功，则返回 `FeatureSourceLayer` 的 `FeatureSource`。如果转换失败，则返回 `NULL`。