Add a pack.sh script who pack image in a single one.

ImageMagick required.
Makefile generate packed textures when needed.
Load and render sub-textures as normal textures.

It's a little bit bugged but it's work.
Fix a bug : textures were padded too much on generation.
Switch textures only if needed
Compute texture offset when generating packed textures.

Render is now correct.
Merge branch 'opengl' of github.com:needs/anko into opengl