HUD viewport's background gets moved to front on reconnect

[squeek502]

Mirrored from the old project tracker on the FF forums:

Not totally sure that this is an issue still, but I got it while testing a bunch of stuff when trying to figure out how to get the mouse to work on the HUD viewport. I believe it's a side-effect of calling MakePopup() from the HUD viewport, which is a base engine function that we don't have the source code to, but is necessary for SetMouseInputEnabled to have any affect.

What was happening:

• Start a local server
• Enter HUD editing mode
• Disconnect without leaving HUD editing mode
• Start a new local server
• HUD viewport background would draw over everything, making the entire screen dark and with no other VGUI shown at all. Simply clicking would make it go away and show the MOTD vgui as normal.

It was sporadic and I've not gotten it with the code that I committed, but it could be a problem.