Update mob fire vfx a bit (#95199)

## About The Pull Request

1. Removes the particle effects from mob fire. 

2. Fire lighting now scales with how "on fire" you are. 

3. Adds an emissive effect to mob fire. 

4. Slightly increased the number of firestacks needed for the more
intense effect (by 10%) (3->5)

5. Fixes the singular broken frame in monkey fire sprite. 

6. Xenomorphs now use the generic fire sprite, adjusting the offset for
larger xenos. Closes #12605 .

## Why It's Good For The Game

1. The effect just doesn't look good as mobs move and rotate
unfortunately. We could probably revisit it in the future when particle
frameworks are more developed, but in the meanwhile watching all of a
mob's embers just twist 90 degrees as they flop to the ground looks
jarring.

2. Produces a neat effect of the light dying out as you get closer to
extinguishing a fire.

3. Adds some bloom to fire, and keeps fire visible as the light dies out
(as the firestacks decrease). I think it looks neat, though I'm not sure
how... realistic? It is in practice?

4. You scarcely see the "low stack" fire vfx due to the low threshold it
was set to. This change is intended to make it slightly more common.

5. Bugfix. It was not offset correctly.

6. Bugfix. I didn't sprite anything unique for them - I just made them
use the generic fire overlay.

<img width="336" height="142" alt="image"
src="https://github.com/user-attachments/assets/ceeac35d-912c-4ad1-9d3b-4384ffdb905b"
/>

## Changelog

🆑 Melbert
del: Removed ember particle effects from mob fire 
add: Mob fire brightness now scales with how "on fire" it is - the more
fire, the brighter the light
add: Mob fire now has a bloom effect and glows a bit
add: The threshold that human fire sprites update from "small" to "big"
has been increased (by 10%)
fix: Fixed broken frame in monkey fire animation
fix: All Xenomorphs now use the generic fire sprite
fix: All Xenomorphs no longer lose their fire sprites when resting
fix: Large Xenomorphs now offset their fire overlay to the middle of
their sprite
/🆑

code/modules/mob/living/basic/basic.dm

@@ -305,16 +305,7 @@
 	adjust_bodytemperature((maximum_survivable_temperature + (fire_handler.stacks * 12)) * 0.5 * seconds_per_tick)
 
 /mob/living/basic/get_fire_overlay(stacks, on_fire)
-	var/fire_icon = "generic_fire"
-	if(!GLOB.fire_appearances[fire_icon])
-		GLOB.fire_appearances[fire_icon] = mutable_appearance(
-			'icons/mob/effects/onfire.dmi',
-			fire_icon,
-			-HIGHEST_LAYER,
-			appearance_flags = RESET_COLOR|KEEP_APART,
-		)
-
-	return GLOB.fire_appearances[fire_icon]
+	return make_generic_fire_overlay()
 
 /mob/living/basic/put_in_hands(obj/item/I, del_on_fail = FALSE, merge_stacks = TRUE, ignore_animation = TRUE)
 	. = ..()
@@ -351,4 +342,3 @@
 		return TRUE
 	else
 		return FALSE
-