Channel autoscrolling fails with lastpos_line enabled

[Sajime]

When a user scrolls up in the channel window, and then back down to the bottom, the window should resume autoscrolling as new lines are displayed. This fails, however, if the "last position" marker is shown at the very bottom of the window. The user must wait until another line of text is displayed after the marker, and then manually scroll to the bottom, to make autoscrolling work again.