Replaces /image with /mutable_appearance, where appropriate (#26518)

In cases where you're creating an image to use as an overlay, it makes more sense to use a mutable_appearance if you can. The image will create a static appearance for not just the image but also each intermediate step if you change vars along the way. The mutable appearance avoids this unnecessary and expensive process. The only situation that requires an image instead of a mutable_appearance is if the overlay is supposed to be directional. MA's ignore direction while images don't. I dunno why, probably another BYOND-ism.

I added a convenience function, mutable_appearance(), designed to emulate image(). Also went ahead and set the default plane of /mutable_appearance to FLOAT_PLANE because it's fucking 0 by default.

Several overlays that were image() calls were changed to just text strings when I could. overlays += "string" has the same result as overlays += image(icon, "string") and saves a proc call.

code/modules/detectivework/evidence.dm

@@ -47,15 +47,12 @@
 
 	icon_state = "evidence"
 
-	var/xx = I.pixel_x	//save the offset of the item
-	var/yy = I.pixel_y
-	I.pixel_x = 0		//then remove it so it'll stay within the evidence bag
-	I.pixel_y = 0
-	var/image/img = image("icon"=I, "layer"=FLOAT_LAYER)	//take a snapshot. (necessary to stop the underlays appearing under our inventory-HUD slots ~Carn
-	img.plane = FLOAT_PLANE
-	I.pixel_x = xx		//and then return it
-	I.pixel_y = yy
-	add_overlay(img)
+	var/mutable_appearance/in_evidence = new(I)
+	in_evidence.plane = FLOAT_PLANE
+	in_evidence.layer = FLOAT_LAYER
+	in_evidence.pixel_x = 0
+	in_evidence.pixel_y = 0
+	add_overlay(in_evidence)
 	add_overlay("evidence")	//should look nicer for transparent stuff. not really that important, but hey.
 
 	desc = "An evidence bag containing [I]. [I.desc]"