Add to xray style handling for fallback/default raster layername. Adjust fallback raster layername to be underscorey.
Assume the image source is not very self-aware.
Merge branch 'master' into raster-default

Conflicts:
	backend.js
Use opacity 1 always for rasters.
Bump up meanError for platform differences.
Merge pull request #65 from mapbox/raster-default

Raster default