Adds a game option to toggle the MultiZ parallax effect for a player (#71005)

Adds a game option, similar to Ambient Occlusion, to toggle MultiZ
scaling, ie the MultiZ parallax effect, which I'm calling it in the
option since that's more clear.

I mainly am curious if this is the cause of significant lag on icebox,
or if it's just the use of a huge number of planes in general.

I don't experience any client lag locally (I'm guessing lemon doesn't
either) so **I mainly intend this to be Test Merged** for people to find
out if the the actual plane.transform being set is the cause.

code/modules/client/preferences/multiz_parallax.dm

@@ -0,0 +1,16 @@
+/// Whether or not to toggle multiz parallax, the parallax effect for lower z-levels.
+/datum/preference/toggle/multiz_parallax
+	category = PREFERENCE_CATEGORY_GAME_PREFERENCES
+	savefile_key = "multiz_parallax"
+	savefile_identifier = PREFERENCE_PLAYER
+
+/datum/preference/toggle/multiz_parallax/apply_to_client(client/client, value)
+	// Update the plane master group's Z transforms.
+
+	var/datum/hud/my_hud = client.mob?.hud_used
+	if(!my_hud)
+		return
+
+	for(var/group_key as anything in my_hud.master_groups)
+		var/datum/plane_master_group/group = my_hud.master_groups[group_key]
+		group.transform_lower_turfs(my_hud, my_hud.current_plane_offset)