(Hopefully) Lessens preferences and manifest injection lag (#30596)

* first step to fixing a potential problem is identifying it

* woops

* kk

* mso is already doing this kek

* yeah lets do that too.

* replicated code

* wups

* wups

* better

* gibe the merge b0ss

* cyberboss

* Update dummy.dm

code/modules/mob/dead/new_player/preferences_setup.dm

@@ -34,7 +34,7 @@
 				return
 
 	// Set up the dummy for its photoshoot
-	var/mob/living/carbon/human/dummy/mannequin = new()
+	var/mob/living/carbon/human/dummy/mannequin = generate_or_wait_for_human_dummy(DUMMY_HUMAN_SLOT_PREFERENCES)
 	copy_to(mannequin)
 
 	// Determine what job is marked as 'High' priority, and dress them up as such.
@@ -82,4 +82,4 @@
 	CHECK_TICK
 	preview_icon.Scale(preview_icon.Width() * 2, preview_icon.Height() * 2) // Scaling here to prevent blurring in the browser.
 	CHECK_TICK
-	qdel(mannequin)
+	unset_busy_human_dummy(DUMMY_HUMAN_SLOT_PREFERENCES)