when a gplanarity window is resized in a way that vertices leave the
visible area, the vertices are pushed inwards, destrying game progress.

for window managers that ignore size hints, this is easy to reproduce:
just make the window sufficiently smaller after starting a level.

for window managers that respect size hints, the steps are:

* fullscreen the window
* zoom in, maybe move points around so more points are near the edges
* unfullscreen the window
* the outer points now stick to the border of the window
* fullscreening the window again leaves them at the old window's outline

in practice, this happens if one uses a tiling window manager (even if
it respects size hints) as resizes occur frequently there, and only if
one has made good progress on a difficult level.

i'm not sure which severity to assign, as it is a form of data loss but
occurs only in rare situations.