Previous PR to reduce fitsview zoom memory use created pixelated rendering for some overlays
with large amount of zoom--e.g. when using a small view (e.g. around a focus view with subframe enabled.
Here we employ a different rendering strategy when the view has a smaller number of pixels (and thus
memory use shouldn't be as much of an issue) so that the rendering is smoother.

Load fits images, try all the overlays, zoom in and out.
Overlays should not change with images greater than a megapixel.
Try this with various views with subframing (e.g. focus), and the rendering should be less pixelated.