Fix switching to a larger puzzle size under GTK 2.

Fix switching to a larger puzzle size under GTK 2.

Commit 8b491946e had a bug: configure_area stopped doing most of its
work if the new size already matched fe->w and fe->h, but in fact the
GTK2 resize_fe() _already_ set up fe->w and fe->h for the new size. I
managed not to notice, because I checked it all worked on GTK 3 but
only tested resizing to a _smaller_ puzzle on GTK 2. Ahem.

Now we don't change fe->w and fe->h at all until configure_area is
called. Also, we initialise them to dummy values at setup time, so
that configure_area won't compare the new size with uninitialised
data.