Changes resulting in 2006-06-20 Enhancement 4

When the playback option was added, the DisplayBoard function was called quite freqently.

The basic design of that function was
CLS
Build an empty chess board
Populate it with pieces

This was quiet appropriate during play of the game as it erased clutter elsewhere on the screen and anyway was not noticable to the player.

But during playback, there is no clutter and the clearing of the screen after every move was noticeable, even irritating, to the viewer.

This patch introduced a new shared variable, pbm (playback mode) and patch DisplayBoard so that during playboard
CLS <--- This is skipped except on first call
Build an empty chess board <--- This is skipped except on first call
Populate it with pieces <--- This is modified

The modification to "populate" is to populate EVERY square with either a piece/pawn or spaces if there is no piece/pawn.