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.
Asteroid turfs are ```/turf/simulated/mineral``` and ```/turf/simulated/floor/plating/asteroid```
so yes