function computeMirrorDimensions({source, ...args}) {
  return withPromise((resolve) => {
    const sourceRect = source.getBoundingClientRect();
    resolve({source, sourceRect, ...args});
  });
}
Computes mirror dimensions based on the source element
Adds sourceRect to state
@param {Object} state
@param {HTMLElement} state.source
@return {Promise}
@private