Easy texture packs support

[y.petremann]

For what I've seen now, to make a texture pack, we need to choose between these options:

• Overwriting Base and Mods stuff directly.
• Overwriting by making a mod.

Making a mod for that is great, but we need to change every texture definitions for textures we have modified.
We also need to modify size definition in case we have just scalled the texture

So :

1. We should have a function to explicitely replace textures from a mod with those in the given folder ()
2. Instead of giving the size of a sprie, it should be more interesting to give the number of rows and colums of the texture, so scaling would not be a problem.
3. In some case like for gui.png and cursor-boxes.png, giving area of the texture from 0 to 1 in each direction could be a solution.
4. The two last point could let precalculating the actual values from them, so the game engine could handle them as now.