The rasinfo structure provides information to the system about the
location of the bitmap as well as the positioning of the display area as a
window against a larger drawing area. Use the following steps to prepare
the RasInfo structure:

    /* Initialize the RasInfos. */
    rasInfo.BitMap = &bitMap;  /* Attach the corresponding BitMap.       */
    rasInfo.RxOffset = 0;      /* Align upper left corners of display    */
    rasInfo.RyOffset = 0;      /* with upper left corner of drawing area.*/
    rasInfo.Next = NULL;       /* for a single playfield display, there
                                * is only one RasInfo structure present  */

The system may be made to reinterpret the rxoffset and ryoffset values in
a viewport's rasinfo structure by calling scrollvport() with the address
of the ViewPort. Changing one or both offsets and calling ScrollVPort()
has the effect of scrolling the ViewPort.