Compass retina sprite mixin

_retinaSprite.scss

/*
    @author: Matt Jewell - mattjewell.co.uk
    
    Borrows HEAVILY from  thulstrup / compass-retina-sprites.scss  https://gist.github.com/thulstrup/2140082
        but ditches the media queries and serves retina to all

*/
// SET UP SPRITES
@import "autosprite/*.png";
$sprite-map: sprite-map("autosprite/*.png");
.sprite-spacing-retina-hack { @include sprite($sprite-map, sprite-spacer); }
/*
    By including a 300x1px image in the sprite map, we know it's width will be 300px
     so we can set the background-width to 150px
*/
$sprite-background-width: 300px;
@mixin retina-sprite ($name) {
    background-image: sprite-url($sprite-map);
    @include background-size($sprite-background-width/2);
    $ypos: round(nth(sprite-position($sprite-map, $name), 2) /2);
    background-position: 0 $ypos;
}
@mixin non-retina-sprite($name) {
    @include sprite($sprite-map, $name);
}