Sprite mixins from json2css' sass template + a retina sprite mixin.
grunt-spritesmith uses json2css templates, but I'm using a slightly modified template and multiple spritesheets, so I'm extracting these mixins into a bower component.
Note: The retina mixins depend on the 2x spritesheet being exactly twice the size of the 1x spritesheet. That means individual 1x sprites should be exactly half the size of 2x sprites before combining them with grunt-spritesmith.