Optimize icon sizes for all resolutions, default is 2x

[sensescape]

trafficstars

@bcamper @nvkelso

This refers to our discussion at Mapzencon about setting sizes for icons on the stylesheet and avoiding any resizing in the stylesheet if possible (only resizing on the stylesheet for lower resolutions) to avoid any distortion of iconographic symbols.

[bcamper]

Looking at this more I discovered an issue with defaulting sprite sizes: without some additional metadata indicating the intended native resolution for a sprite-sheet, we can't keep the size consistent across retina and non-retina displays. In the meantime we can keep specifying size in the scene, but I would like to solve this so that we can automatically size things appropriately, and select different resolution sprite-sheets based on current display density. The simplest solution for this would be something like a density property on the sprite-sheet, but I also want to think through how we specify a list of images (that are all logically the same set of sprites) before implementing new syntax.

cc @blair1618