Now that we use gdk_window_create_similar_surface() to make our backing
surface, most of the code to handle scale factors is unnecessary and
indeed slightly harmful. gdk_window_create_similar_surface() creates a
surface suitable for use as backing for a window, so it already has the
scale factor applied. This means that it should be sized in nominal
pixels, we can paint to it in nominal pixels, and the fact that it has
the same extra resolution as the actual window is entirely transparent
to us.
Now the only reason we pay attention to the scale factor at all is to
detect changes and use them as a prompt to re-create the backing
surface.
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