http: optimize HTTP request thread we don't need a mutex at all, in fact all we need is an atomic boolean to signify whether the thread is cancelled. curl options are now for the most part handled by a separate function to keep them in sync between non-threaded and threaded implementations

// Opacity

.opacity(@opacity) {
  opacity: @opacity;
  // IE8 filter
  @opacity-ie: (@opacity * 100);
  filter: ~"alpha(opacity=@{opacity-ie})";
}