[KIconDialog] Render SVGs with proper devicePixelRatio

SVGs can scale indefinitely, so render them crisply.

Also move all the resizing and padding into the non-SVG code path since for SVG we create the QImage ourselves so it will always have the correct dimentions, making the code easier to follow.

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Verified that SVGs are rendered crisp and that PNGs are unchanged.

I don't understand why without specifying bounds on QSvgRenderer::paint the pixmap is rendered too big (i.e. I only see a quarter, as if QSvgRenderer didn't do high dpi properly...)

@davidedmundson @cfeck @cullmann

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