[Planner] misalignment in Gantt view

[James Riordan <rij zurich ibm com>]

When viewing my project in the Gantt view with planner version 0.13 on a
few different Gentoo Linux systems (but not on a Debian system), there
is a misalignment between the task name and the graphical representation
which increases from top (first task) to bottom.  After about twenty
items, the misalignment is already off by about two entire rows and
makes editing rather difficult.

It sort of looks like an off-by-one problem someplace where Debian and
Gentoo differ in versions (gnome libs?).  I fear I am not so good with
GTK and cannot seem to find where are heights for the graphical pane are
computed.  Can anybody provide a hint?

Thanks,
James