Add FLOOR_OVERLAY_PLANE (and use that for wetness)

code/__DEFINES/layers.dm

@@ -7,6 +7,7 @@
 #define PLANE_SPACE_PARALLAX -90
 
 #define FLOOR_PLANE -2
+#define FLOOR_OVERLAY_PLANE -1.5
 #define GAME_PLANE -1
 #define BLACKNESS_PLANE 0 //To keep from conflicts with SEE_BLACKNESS internals
 

code/game/turfs/simulated.dm

@@ -53,7 +53,7 @@
 				wet_overlay = image('icons/effects/water.dmi', src, "ice_floor")
 			else
 				wet_overlay = image('icons/effects/water.dmi', src, "wet_static")
-		wet_overlay.plane = MID_TURF_LAYER
+		wet_overlay.plane = FLOOR_OVERLAY_PLANE
 		overlays += wet_overlay
 	if(time == INFINITY)
 		return