Improve pane position saving code

We should test for being in PANE_THREE mode in save_pane_position() and
replace the if()'s testing against window geometry. This eliminates some
code duplication and, more importantly, save_pane_position() is also
called from save_window_geometry() whose calling pattern is not obvious.

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>