Optional show dimmed background behind dialog txt with panel

[Nash]

Going to self-assign this, putting it here so it won't be forgotten.

When you assign a panel to a Strife dialog (like Rowan's face for example), the dimmed background behind the dialog text disappears. Optionally allow the dim to continue to be drawn even with a panel graphic assigned, as depending on what kind of artwork is used for the panel, text readability may be compromised.

As for how and where this would be exposed, I haven't thought it through yet (Some bool on ConversationMenu? MAPINFO key for people who don't want to use ZScript? Something else?)

[Graf Zahl]

Definitely an option on the dialogue node and a global switch. And a CVAR would also be convenient for quick testing.