* Made wet floors use a static overlay.
 * Standardized the wet floor code, it is now called MakeSlippery(var/wet_setting)
 * Added the wet floor static overlay, which is just a single frame of the usual animation.
 * Made the floor randomly dry (by a few seconds apart), for visual effect.
Merge pull request #1793 from Giacom/wet_static_overlay

Made wet floors use a static overlay.