Duke3D: Fix viewscreens being permanently deactivated after loading a save

Merged Dino Bollinger requested to merge dibollinger/eduke32:vscr_fixup into master

Also decreases MAXUSERTILES to (MAXTILES - 256) to ensure the tilespace used by the viewscreens isn't encroached on by user tiles.

I checked in Fury and decreasing MAXUSERTILES seems to be compatible with old savegames.

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