Fix disappearance of premoved piece

Fix disappearance of premoved piece

When a premove was done by dragging, (which for expected recaptures would
be the only way, as click-click moving selects the intended target),
the moved piece would disappear from view. This because the from-square
would not be redrawn (in normal move execution it would remain empty).
This could have been an XBoard-only problem. In any case We now force a
complete redraw of the board, which is not very elegant, but should always
fix it.