Clear image before drawing new one to prevent pages of different sizes being drawn over each other

[GKFX]

Screenshot of the problem below: Note that this does introduce a moment where there is no image and so the screen flashes black.

[dsanson]

In my own use, I'd rather occasionally have this problem than constantly have the momentary flash of black. I suppose it would be possible to keep track of the size of the previous image, and only do this if the new image is smaller...