Save threadID info in Position

This is the best place because when we split we do a
copy of the position and also threadID, once set in a
given position, never changes anymore.

Forbid use of Position's default and copy c'tor to avoid
nasty bugs in case a position is created without explicitly
setting the threadID.

No functional change.

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

src/material.cpp

@@ -403,7 +403,7 @@ Key EndgameFunctions::buildKey(const string& keyCode) {
         s << char(upcase? toupper(keyCode[i]) : tolower(keyCode[i]));
     }
     s << 8 - keyCode.length() << "/8/8/8/8/8/8/8 w -";
-    return Position(s.str()).get_material_key();
+    return Position(s.str(), 0).get_material_key();
 }
 
 const string EndgameFunctions::swapColors(const string& keyCode) {