The Change the Viewport Associated with a Virtual Display routine
    changes the size of an existing viewport in a virtual display.
    The text currently in the viewport is remapped to fit the new
    dimensions.

    Format

      SMG$CHANGE_VIEWPORT  display-id [,viewport-row-start]

                           [,viewport-column-start]

                           [,viewport-number-rows]

                           [,viewport-number-columns]