ChangeTurf tweaks (#4036)

changes:

Lighting's ChangeTurf() override has been merged into the base ChangeTurf() proc.
Turfs can now set an alternate openturf type to be used instead of the default one when ChangeTurf(/turf/space) or ChangeToOpenturf() is used.
Added ChangeToOpenturf() helper.
Added a missing dig icon state to the rocky ash DMI.
Fixed an issue where the above var was not properly carried over on ChangeTurf().
Fixed an issue where Z-mimicing turfs would not be notified of their below turf ChangeTurf()ing unless they were an openspace.

code/modules/mining/mine_turf_types.dm

@@ -12,6 +12,7 @@
 			/turf/simulated/lava,
 			/turf/simulated/mineral
 	)
+	openspace_override_type = /turf/simulated/open/chasm/airless
 
 // Custom behavior here - we want smoothed turfs to show basalt underneath, not lava.
 /turf/simulated/lava/get_smooth_underlay_icon(mutable_appearance/underlay_appearance, turf/asking_turf, adjacency_dir)
@@ -34,6 +35,7 @@
 	light_color = LIGHT_COLOR_LAVA
 	smooth = SMOOTH_FALSE
 	canSmoothWith = null
+	openspace_override_type = /turf/simulated/open/chasm/airless
 
 	footstep_sound = "concretestep"