GdkPixbuf
=========

To load the board image the following is done:

- the image is located in a PGN file with an alpha channel
- the image is loaded into memory as two GdkImages, one for the image
  and one for the alpha channel (as graytones, black= transparent,
  white= opaque) [module PNG]
- the images are stretched [module CLIENT_IMAGE]
- the images are sent to the X Server [module CLIENT_IMAGE]
- the pieces are drawn using boolean operators: [module SERVER_IMAGE]

  ----------------         ------------             -----
  BACKGROUND IMAGE         ALPHA CHANNEL            IMAGE
  ----------------         -------------            -----
         |                   |    |                   |
         `------AND NOT------'    `-------AND---------'
                   |                       |
		   `-----------OR----------'
                               |
			       |
		        --------------
  		        COMPOSED IMAGE
			--------------


To do that with GdkPixbuf, the alpha channel must be separated from
the images.  It's not enough to used gdk_draw_pixbuf(), because that
would load the images from X client space everytime a piece is drawn. 

How can two X GdkPixmaps be generated from a PGN file that has an
alpha channel using GdxPixbuf?

