Allow WinBoard to use bitmaps with alpha channel

Allow WinBoard to use bitmaps with alpha channel

When the primary image of a piece (*o.bmp or *s.bmp) has a format with
32 bit/pixel, it is assumed to be in ARGB format, with transparent
background. It is then drawn over the board accordingly by new code
(using the Win API AlphaBlend routine). This code ignores the allWhite
and flipBlack options for the moment.
  The cascade for loading bitmaps has been changed as well: WinBoard
now first tries to load an image with name like piece0_49o.bmp from the
pieceImageDirectory if the traditional name (like p49o.bmp) is not found,
before falling back on built-in images.