A mechanism for scraping a game when going through moves.

[Tomasz Sobczyk]

We may want to scrap positions collected from a game before commiting it for some reason.
To do it we have to be able to always pack all moves into the same vector and in the case of a rollback pop some of them. Currently a position vector can be commited when in the middle of parsing a game. A reasonable limit on the number of moves could be added, like 1024.