move the internal preview to canvas (#18930)

* move the internal preview to canvas

* use a helper

* basic readme prep

* .

* .

* transform  better

* byond can't handle icon states of large files...

* move fully to the rust version

* fix typing

* just wtf. NO

* fix the legacy path

* keep it all centralized

* tiny oversight

* unit test rewrite

* rename

* fix bad layering

* fixes a bunch of layering issues

* artifact

* move crash to log_runtime

* missing overlay warning

* why not

code/game/objects/structures/barsign.dm

@@ -8,7 +8,7 @@
 	var/cult = 0
 
 /obj/structure/sign/double/barsign/proc/get_valid_states(initial=1)
-	. = cached_icon_states(icon)
+	. = icon_states_fast(icon)
 	. -= "On"
 	. -= "Nar-sie Bistro"
 	. -= "Empty"