Retina 対応 mixin

## gistfile1.scss
$spriteName: sprite-map("folder/*.png", $spacing: 5px);

@mixin sprite-hoge($name){
  background-image: $spriteName;
  background-repeat: no-repeat;
  width: image-width(sprite-file($spriteName, $name)) / 2;
  height: image-height(sprite-file($spriteName, $name)) / 2;
  $ypos: round(nth(sprite-position($spriteName, $name), 2) / 2);
  background-position: 0 $ypos;
  background-size: round(image-width(sprite-path($spriteName)) / 2) round(image-height(sprite-path($spriteName)) / 2);
}

.no1{
  @include sprite-hoge($name);
}