Change color argument of square_is_weak()

Pass the color for which the square is to be
considered weak, not the opposite.

It is more natural and intuitive in this way.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/position.h

@@ -483,7 +483,7 @@ inline bool Position::pawn_is_passed(Color c, Square s) const {
 }
 
 inline bool Position::square_is_weak(Square s, Color c) const {
-  return !(pieces(PAWN, c) & outpost_mask(opposite_color(c), s));
+  return !(pieces(PAWN, opposite_color(c)) & outpost_mask(c, s));
 }
 
 inline Key Position::get_key() const {