Fix screen swap when using alternate buffer.

Summary:
The circular_offset should not be set to 0 because the first line from
screen2 is copied to the offset line in screen.

Test Plan:
Try "man echo", scroll down and exit. The screen is not restored
properly.

Reviewers: billiob

Reviewed By: billiob

Differential Revision: https://phab.enlightenment.org/D345

src/bin/termpty.c

@@ -1509,7 +1509,6 @@ termpty_screen_swap(Termpty *ty)
                    &(TERMPTY_SCREEN(ty, 0, y)),
                    &ty->screen2[y * ty->w]);
      }
-   ty->circular_offset = 0;
    ty->altbuf = !ty->altbuf;
 
    if (ty->cb.cancel_sel.func)