Fixes the rolling fog effect (#20820)

Separates the rolling fog effect into its own type rather than relying
on varedits, shifts it up a few layers so it renders ingame. Currently,
it doesn't appear at all in the holodeck settings it's used in.

code/game/objects/effects/decals/misc.dm

@@ -28,6 +28,14 @@
 	anchored = 1
 	layer = OBJ_LAYER
 
+// Fancy animated effect used for the holodecks.
+/obj/effect/decal/rolling_fog
+	name = "rolling fog"
+	icon = 'icons/effects/props/holodeck/biesel/32x32.dmi'
+	icon_state = "fog_roll"
+	anchored = 1
+	layer = ABOVE_PROJECTILE_LAYER
+
 /obj/effect/decal/fake_object/Initialize(mapload)
 	.=..()
 	appearance_flags &= ~TILE_BOUND