Diagonal smoothing fixes & Shuttle Smoothing Tweaks (#2466)

Ports a refactor of diagonal smoothing underlays from /tg/, fixes a bug where diagonal smoothing did not work on shuttle walls, and updates shuttle walls to use the generic smoothing instead of mapped in icon states.

Fixes issues with shuttle corners not showing the correct underlay.

code/game/objects/structures/window.dm

@@ -479,13 +479,21 @@
 /obj/structure/window/shuttle
 	name = "shuttle window"
 	desc = "It looks rather strong. Might take a few good hits to shatter it."
-	icon = 'icons/obj/podwindows.dmi'
-	icon_state = "window"
+	icon = 'icons/obj/smooth/shuttle_window.dmi'
+	icon_state = "shuttle_window"
 	basestate = "window"
 	maxhealth = 40
 	reinf = 1
 	basestate = "w"
 	dir = 5
+	smooth = SMOOTH_TRUE
+	can_be_unanchored = TRUE
+
+/obj/structure/window/shuttle/update_nearby_icons()
+	queue_smooth_neighbors(src)
+
+/obj/structure/window/update_icon()
+	queue_smooth(src)
 
 /obj/structure/window/reinforced/polarized
 	name = "electrochromic window"