Adds dirt overlays for walking on simulated turfs

Credit to Comma and DJSnapshot at bay12.

This will make an overlay of dirt appear the more you walk on tiles. It will slowly fade in the more the tile is walked on.  Can be cleaned like pre-made dirt decals.
1 - Id prefer if this all was moved to floors, coz currently you will be dirtying walls if you accidently teleport into them
2 - It should be checking for gravitation or something, coz i doubt you make floors dirty but flying above them