QQuickBorderImage: fix the implicit size of scalable images on high DPI

The implicit size was the image's original size divided by its device
pixel ratio. That's correct for an @2x image, whose original size is in
image pixels. But a scalable image without sourceSize is rendered for
the device pixel ratio, while its original size is its natural size, so
e.g. an SVG ended up with half its natural size on a screen with DPR 2.

Use the size of the loaded image instead, as QQuickImage does.

Fixes: QTBUG-151330
Pick-to: 6.12
Change-Id: I128fdf1bc0d789f241dbfbcc2d6b22bb864a9046
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>