Change the way tilesets with margin/padding are handled

[ikskuh]

Right now, VersaTile modifies the source texture on import. This may be unwanted behaviour if the user wants to rely on the original texture' setup.

Change it so the user can choose between a modified texture and a raw handling.

pro for unmodified:

• Easier handling when models share the texture
• May be easier for the artist

pro for modified:

• User can spawn tiles larger than a single sprite, even when the source material has padding and margin
• A bit easier handling in texture mapping