Fix loading of engine-defined PGN games

Fix loading of engine-defined PGN games

Somehow there could be disagreement over what the official opening
position of an engine-defined variant was during loading of the game.
It then refused Betza-defined castling, which tests the corner pieces
based on this initial position. We now assume the FEN tag, which such
a PGN game will always contain, holds the official opening position,
so that castling will always be assumed possible (if there is a corner
piece).