Fix loading 2x images in Rich Text Documents

There are some issues preventing 2x images from loading, mainly incorrect
testing for exist() by passing an url, which always reports ‘false’.

Task-number: QTBUG-36383
Change-Id: I1e8e5a91b01f1a4ddd3559c2e860db5bc41908ce
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>