Secret Marbles

This mod adds the unused marble skins into the pool of texture that a marble can have, with no modification of the textures.

Notes:

This mod adds 6 new marble types to the marble texture pool, all of which exist in the game files and have not been modified in this mod but are not used by naturally generated marbles by default.
There is a 12/20 chance of any generated marble to be a Zebra, and a 1/20 chance for all of the 8 other marble types.