Repacking spritesheets to be used with SDL2

This repacks the spritesheets using a slightly different packing algorithm.
As a first constraint, it will try to use the smallest power of 2 as upper bound for width/height. 
This will address problems with overly long or height images, see https://github.com/dorkster/flare-engine-next/issues/4 for details.

Also more packing algorithms are added, so we can expect an minor improvement of packing. benchmarks showed we'd get maybe half a percent less area.