CGE2: Add extra clipping to Bitmap::hide
CGE2: Add workaround for the missing dialogs in room 21 (concert)
CGE2: Replace NULL with nullptr everywhere.
CGE2: Hopefully fix memory leak problems regarding _ext.
CGE2: Fix Hero::setCurrent().

This fixes a crash regarding the mechanic on scene 6.
CGE2: Fix another probable memory-leak source.
CGE2: Add extra clipping to Bitmap::hide