Fix issues with Engine.Viewport being wrong during RenderCore

[RedFlames]

...By not messing with it and instead making FakeRT bigger when there are black bars...

This appears to work although I had one weird big crash when I was resizing the window. I'm unsure if maybe I just had really bad timing with resizing as CNet was (dis-)connecting or what, I dunno if that crash was caused by what I did here at all.