Adds Reflection to VV Modify Transform (#4907)

# About the pull request

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As it says on the tin. Just gives the option to flip transform matrixes
in VV Modify Transform.

# Explain why it's good for the game
More shenanigans

# Testing Photographs and Procedure
Tested on brief chairs


# Changelog
:cl:
admin: VV 'Modify Transform' can now be used to flip objects.
/:cl:

code/game/atoms.dm

@@ -723,7 +723,7 @@ Parameters are passed from New.
 		usr.client.cmd_admin_emp(src)
 
 	if(href_list[VV_HK_MODIFY_TRANSFORM] && check_rights(R_VAREDIT))
-		var/result = tgui_input_list(usr, "Choose the transformation to apply","Transform Mod", list("Scale","Translate","Rotate"))
+		var/result = tgui_input_list(usr, "Choose the transformation to apply","Transform Mod", list("Scale","Translate","Rotate", "Reflect X Axis", "Reflect Y Axis"))
 		if(!result)
 			return
 		if(!result)
@@ -749,7 +749,22 @@ Parameters are passed from New.
 					return
 				var/matrix/base_matrix = matrix(base_transform)
 				update_base_transform(base_matrix.Turn(angle))
-
+			if("Reflect X Axis")
+				var/matrix/current = matrix(base_transform)
+				var/matrix/reflector = matrix()
+				reflector.a = -1
+				reflector.d = 0
+				reflector.b = 0
+				reflector.e = 1
+				update_base_transform(current * reflector)
+			if("Reflect Y Axis")
+				var/matrix/current = matrix(base_transform)
+				var/matrix/reflector = matrix()
+				reflector.a = 1
+				reflector.d = 0
+				reflector.b = 0
+				reflector.e = -1
+				update_base_transform(current * reflector)
 		SEND_SIGNAL(src, COMSIG_ATOM_VV_MODIFY_TRANSFORM)
 
 	if(href_list[VV_HK_AUTO_RENAME] && check_rights(R_VAREDIT))