Rounded concave corners for playable area of board

[trichoplax]

To be consistent with the DragonFjord board, the area where the pieces go should have rounded corners. Currently only the convex (outward) corners are rounded. The concave (inward) corners should also be rounded, such that they do not touch the sharp corners of the nearest grid square. This will allow placement of a piece that fits around the concave corner without the piece overlapping the outside area of the board.