qml: preserve aspect ratio in `RoundImage.qml` and provide painted size

Turns out we can actually use fillMode, at least PreserveAspectFit and Stretch. I'm not sure about tiling, it may not be supported so I put a note there.

I preferred to use PreserveAspectFit instead of PreserveAspectCrop for several reasons:

• We don't need clipping (clip: true) (https://doc.qt.io/qt-6/qml-qtquick-image.html#fillMode-prop). We can not use clipping here, that breaks batching. We could modify the shader and have a custom sub-rect, for that 1) we can not use custom vertex shader because it breaks batching in ShaderEffect 2) we can do it in the fragment shader, but that needs multiple branches and would make it look ugly. Since QQuickImage also requires clipping for PreserveAspectCrop, I have less desire to implement it.
• We keep all the cover content. With PreserveAspectCrop, part of the cover would be cropped.
• Playlist delegate also have been using PreserveAspectFit for a long time, for these reasons.

I have tested this with cover images that are not square.

The leftmost cover is thin (height > width), the center cover is fat (width > height), and the rightmost cover is square. All of their aspect ratio is preserved with PreserveAspectFit. The shadow also stretches to the aspect ratio, in the future it can be generated with aspect ratio (which should be constant) pre-applied.

Request review @chub.