Icon: keep the aspect ratio of portrait images with roundToIconSize

updatePaintedGeometry() preserved the aspect ratio only for landscape images. A taller-than-wide (portrait) image fell into the else branch and was painted at roundedWidth x roundedWidth, a square, so it was stretched horizontally. This shows up for instance in the Plasma desktop Folder View, whose preview icons use a non-square box, where portrait photo previews came out stretched wide.

Scale the width to preserve the aspect ratio for portrait images too, mirroring the landscape branch. Square images keep the roundedWidth square as before.

Add a regression test using a 40x80 source in a wide box, asserting the painted area stays taller than it is wide.

Context https://discuss.kde.org/t/how-to-disable-desktop-thumbnail-stretch/48762