Fixes papers and the slide projector (#21559)

Papers no longer have double text shown when examining them.
The slide projector projections can now be examined again, and papers
and photos can be looked at properly through it.

code/game/objects/items/devices/slide_projector.dm

@@ -145,6 +145,7 @@
 	anchored = TRUE
 	simulated = FALSE
 	plane = ABOVE_LIGHTING_PLANE
+	mouse_opacity = MOUSE_OPACITY_ICON
 	alpha = 100
 	var/datum/weakref/source