[wwqjgwga] Highlighted pieces and squares

From a given chess position, consider the best line (principal variation) or the best few lines if there are multiple that are equally good.  For every square that is a destination of a piece in the lines, shade that square a certain color.  Squares that get involved only deeper in the line get less shading.  Square that get hit multiple times get darker shading.  We could also allow sophistication in which the shading color differs depending of the color of the piece or pieces that land on it.

Similarly, in the board diagram of the current position, highlight pieces that will move, perhaps darker if they move sooner or a lot, perhaps a different shade if they will be captured.

In general, these highlights annotate the "hot" squares and pieces in the line about to happen (assuming best play) from the current position.  This may be useful for spectating chess.  Previously.