OvrSetBuf procedure

  DECLARATION:  OvrSetBuf(BufferSize : LongInt)

      PURPOSE:  Sets the size of the overlay buffer

         UNIT:  Overlay

      REMARKS:  The value of BufferSize must be as large as, or larger than,
                the initial size of the overlay buffer.  It must also be less
                than or equal to MemAvail + OvrGetBuf.

                OvrSetBuf requires that the heap be empty.  If any dynamic
                variables have allocated using New or GetMem, an error will
                occur.  The procedure InitGraph (from the Graph Unit) will
                allocate memory on the heap, so be sure to call OvrSetBuf
                first.

                Errors are reported through OvrResult; See the error
                messages section for details about OvrResult.