Fallback to use non-retina assets

[mick]

Mapbox-gl-js automatically tries to load retina assets on retina screens, but if those images arent available it should fallback to using the non-retina version of the image.

Example: http://mapmeld.github.io/van-gogh-map/#6.06/39.756/-814.736 fails to load on a retina screen with these errors:

[jfirebaugh]

We have future plans for sprites that would make this unnecessary -- mapbox/mapbox-gl-style-spec#220, mapbox/mapbox-gl-style-spec#222. Outside of changes in that vein, we're not planning to change the behavior of sprite asset requests, so I'm going to close this issue.