fix(spritesmith): split spritesheets in to multiple files to remove the
blurry iOS issue. Fixes
https://github.com/HabitRPG/habitrpg/issues/2510, see
http://goo.gl/IwxdI4 . Note, this commit isn't smart/dynamic, there's
commented-out code for handling dynamic splitting, but I couldn't get it
working - static 4 sheets for now