Fall back on Tile SVG in pieceImageDirectory

Fall back on Tile SVG in pieceImageDirectory

If a -pieceImageDirectory is given, but a needed piece is not found,
and -inscriptions are specified, we first fall back on a general
image White/BlackTile.svg. This can then serve as a background for the
various inscription glyphs, using the latter to distinguish the pieces.
E.g. the Tile image could be a shogi tile, and the inscriptions an utf8
string of kanji, to create Shogi pieces on the fly.