3x support for retina.less mixin

3xmixin.less

.at2x(@path, @w: auto, @h: auto) {
  background-image: url(@path);
  @at2x_path: ~`@{path}.replace(/\.\w+$/, function(match) { return "@2x" + match; })`;
  @at3x_path: ~`@{path}.replace(/\.\w+$/, function(match) { return "@3x" + match; })`;

  @media @highdpi {
    background-image: url("@{at2x_path}");
    background-size: @w @h;
  }

  @media @3xdpi {
    background-image: url("@{at3x_path}");
    background-size: @w @h;
  }
}