Don't refresh backing store on a no-op configure event.

Sometimes, we can get a "configure_area" event telling us that the
drawing area has changed size to the same size it already was. This
can happen when we change puzzle presets in a way that doesn't change
the size, and also sometimes seems to happen as a side effect of
changing the text in the status line.

In that situation, it's a waste of effort - and can cause visible
on-screen flicker - to throw away the window's backing image and
pixmap and regenerate them from scratch. So now we detect a non-resize
and avoid doing all that.

The only thing we retain unconditionally in configure_area is the
midend_force_redraw, because that's the place where a puzzle redraw is
forced when changing presets or loading a new game.

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