GU__SelectScreen (ul_pbar_graphics)

 *  Synopsis:
 *    #include <ul_pbar_graphics:gu.h>
 *
 *    long int GU__SelectScreen( long int displayScreen )
 *
 *  Function intent:
 *    GU__SelectScreen() selects the specified graphics screen.  It determines
 *    if the call is legal based upon the specified screen id and the current
 *    program context (i.e., PA or SA) and returns an error only if an illegal
 *    request has been made.
 *  Function parameters:
 *    long int displayScreen  the requested screen value: GXPA_1 or GXPA_2
 *  Function return value:
 *    CBS_INVARG  invalid screen value
 *    CBS_INVTYP  calling program is not a Primary Application
 *    GXPA_1    previous screen was graphics screen 1
 *    GXPA_2    precious screen was graphics screen 2