screenblank on 4D/70GT

[Mike Muuss]

Use the blanktime() call to set the blanking interval.
Note that on some 4D machines, the parameter may not exceed 2^15,
even though the parameter is supposed to be a LONG.
eg:

        /*
         *  Set an 8 minute screensaver blanking, which will light up
         *  the screen again if it was dark, and will protect it otherwise.
         *  The 4D has a hardware botch limiting the time to 2**15 frames.
         */
        blanktime( (long) 32767L );

Best,
 -Mike