[MIRROR] Fixes a bug with the immerse element [MDB IGNORE] (#23073)

* Fixes a bug with the immerse element (#77585)

## About The Pull Request

What it says on the tin. This runtime was caught downstream in an away
mission unit test where a mob is placed in a water turf.

![image](https://github.com/tgstation/tgstation/assets/13398309/8123321d-8bf4-4186-a63d-944080ad9f8a)

Basically the immerse overlay wasn't being added the very first time it
was created, because it wasn't being returned by
`generate_vis_overlay()`. Derp.

It went unnoticed because the overlays are cached and so every
subsequent `add_immerse_overlay()` works fine. And I guess upstream
doesn't have any mobs that spawn in water during tests so it was never
caught.

Also stops `on_init_or_entered()` from needlessly being called twice in
the case of a movable atom in water being initialized before the water
turf. The signal `COMSIG_ATOM_AFTER_SUCCESSFUL_INITIALIZED_ON` already
sees to that.

## Why It's Good For The Game

Fixes a bug

## Changelog

🆑
fix: fixes immerse overlays not being added the first time a mob enters
a water turf
/🆑

* Fixes a bug with the immerse element

---------

Co-authored-by: Bloop <13398309+vinylspiders@users.noreply.github.com>

code/datums/elements/immerse.dm

@@ -88,6 +88,8 @@
 	RegisterSignal(source, COMSIG_ATOM_ABSTRACT_EXITED, PROC_REF(on_atom_exited))
 	attached_turfs_and_movables += source
 	for(var/atom/movable/movable as anything in source)
+		if(!(movable.flags_1 & INITIALIZED_1))
+			continue
 		on_init_or_entered(source, movable)
 
 ///Stops the element from affecting on the turf and its contents. Called on Detach() or when TRAIT_IMMERSE_STOPPED is added.
@@ -206,6 +208,7 @@
 	vis_overlay.overlay_appearance = overlay_appearance
 
 	generated_visual_overlays["[is_below_water][width]x[height]"] = vis_overlay
+	return vis_overlay
 
 ///This proc removes the vis_overlay, the keep together trait and some signals from the movable.
 /datum/element/immerse/proc/remove_immerse_overlay(atom/movable/movable)