function validateCubemapSrc(src, cb) {
  return validateEnvMapSrc(src, cb, function isEquirectCb() {
    warn('Expected cubemap but got image');
  });
}
Validates six images as a cubemap, either as selector or comma-separated
URLs.

@param {string} src - A selector or comma-separated image URLs. Image URLs
          must be wrapped by `url()`.
@param {function} cb - callback if src is a cubemap.