Refactoring of material-draws adjudication code

Refactoring of material-draws adjudication code

Insufficient mating material is now determined per side individually, by
testing for MatingPotential of each side. If both lack mating potential
the game is adjudicated as draw (if the -materialDraws option is set).

This way MatingPotential() can later also be used to correct losses by
forfeit to draws (which currently is only done when the opponent has a
bare King).

The accuracy of this type of adjudication in Xiangqi has been highly
improved.