_vidblt -- video block transfer

        USAGE

          _vidblt(sseg, soff, dseg, doff, n);
          unsigned sseg, soff, dseg, doff;
          int n;

                sseg - source segment
                soff - source offset
                dseg - destination segment
                doff - destination offset
                n -    number of bytes to BLT

          _vidblt is similar to the lattice movedata() function except
          that it waits for the video retrace signal before performing
          the block transfer.

          _vidblt is a general purpose function that can be used outside
          of the window environment.

        RETURNS

          Nothing

        CAUTIONS

          For use in color systems only.

          _vidblt references wn_sbit.
.