Add option to control downsampling on stretch slider preview.

Currently when the user uses the sliders that control the fitsviewer's stretch parameters, the preview of the stretch shown is coarse (downsampled by 4) in order to speed up display. Jim (@xsnrg) reasonably requested this simple change to allow full resolution previews when the user has the computational resources. Here I add a control in the FITS options section to do that. See this screenshot. https://photos.app.goo.gl/vP2KiiPCnJ2YMxnk7 Setting the Stretch Preview spinbox to 1 would make the preview full resolution, keeping it at 4 would maintain the current default.

To test, set Stretch Preview to 1, and on a fast enough machine, use the stretch sliders. You should see no change when you release the slider. If you have it set to 4, you should see some change in the image when you use a stretch slider after releasing the slider (e.g. it changing from downsampled-by-4 to full resolution).