Force exactly overlayed texture scaling through filename

Force exactly overlayed texture scaling through filename

A filename suffix -NxM.png on a texture image will cause XBoard to scale
the image as if it were the image of a complete board of N files and M
ranks, to make the square size equal to that of the current board,
and adjust the cutting parameters such that contiguous squares are cut
out of it. (If the current board format is larger than NxM, the remaining
space will be padded with white!)
 If N=M=0, N and M are assumed to correspond to the current board size,
and the bitmap will be scaled to fit the height of the board.
 If no suffix is present, the old cut-and-tile algorithm is used. It will
be assumed the texture bitmap is intended for a single square, unless the
pathname contains a file that starts with "xq", in which case it is assumed
that the file contains a Xiangqi board.