gtk: correctly calculate initial window size

For a long time, we've accepted that there's no way to set the size of a widget directly in GTK, and while that is true, it doesn't mean we couldn't try anything better. The RenderSurface rewrite gave us the opportunity to instead make the render surface declare its default size its gtk.WidgetClass.measure implementation, which is then propagated upwards to make GTK derive the correct window size automatically. This has the added benefit of making a lot of bookkeeping unnecessary.

One caveat is that we have to duplicate the window size logic in an apprt, but we can fix that with a subsequent cleanup.

AI disclosure: I let Pi do most of the iterative debugging work and then wrote the actual diff by hand.

Closes #14157