added --ordering=filename 

done in a simplistic way and without tests. 

orders files in the sprite by filename instead of image sizes or areas, so that the ordering is always deterministic. 

this is useful for generating retina and non-retina sprites with different image contents but with the same image filenames. here is an example: 
 
1) generate non-retina sprite using glue
2) generate retina sprite using glue
3) use non-retina sprite image of step 1, retina sprite image of step 2 and css generated by step 2