Outside background may be auto-changed differently than VVVVVV

Reported by Lollipop, see #ved on VVVVVV Discord, 16-12-2018

https://cdn.discordapp.com/attachments/403718283855134720/523727187715686400/GIF.gif

"placing outside backing in M&P and trying to edit it in ved causes it to not feel so good"

Just placing this here so I don't lose track of this.