Hydrate a layer group
Normalizes all the ref layers in a group.

@param {Object[]} layersDef All layers defined in the Mapbox GL stylesheet.
@param {Object[]} layerGroup Subset of layers to be rendered as a group.

@returns {Object[]} An array with the ref layers normalized.
function hydrateLayerGroup(layersDef, layerGroup) {
  const hydrated_group = [];
  for (let i = 0, ii = layerGroup.length; i < ii; i++) {
    // hydrateLayer checks for "ref"
    hydrated_group.push(hydrateLayer(layersDef, layerGroup[i]));
  }
  return hydrated_group;
}