Repack images in fantasycore/alpha_demo except for powers


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.

This does repack the enemies, loot, npc and avatar graphics. This pull request leaves out the powers, because powers have lots of 
transparency usually. And as the repacking also recreates the non-alpha versions, the powers cannot be created with lots of manual fine tuning
for the non-alpha versions.

Everything except for the goblins were repacked automatically. I did a short playtest and could not find mistakes. However I'd like to keep each image repacking in one commit, so we can revert it one by one in case there are minor mistakes in the no-alpha version.

This one is mostly relevant for @dorkster I'd assume, so he can test SDL2 on his netbook.