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