Only switch to Qt::FastTransformation ("showing pixels") at 400% zoom

Right now Gwenview uses Qt::FastTransformation to show individual pixels at any zoom value higher than 100%. This is not optimal and results in poor display for low zoom levels where a smoothed image is more aesthetically desirable. But there are use cases for seeing individual pixels--for example when looking at the details of a line-art graphic or an icon, or examing an image to see its individual colors. These use cases becomes more likely as the zoom level is increased.

Accordingly, this commit changes Gwenview to shop smoothing zoomed-in images at an arbitrarily-chosen threshold of 400% zoom. At that level or above, the image will be unsmoothed and you can see the individual hard-edged pixels.

BUG: 443010 FIXED-IN: 22.04

cc @teams/vdg @teams/usability