Once you have a BitMap set up, you can declare and initialize the RastPort
and then link the BitMap into it.  Here is a sample initialization
sequence:

    struct BitMap bitMap = {0};
    struct RastPort rastPort = {0};

    /* Initialize the RastPort and link the BitMap to it. */
    InitRastPort(&rastPort);
    rastPort.BitMap = &bitMap;


    Initialize, Then Link.
    ----------------------
    You cannot link the bitmap in until after the RastPort has been
    initialized.