QQuickPixmap: detect scalable images in data URLs

Whether an image is loaded for the device pixel ratio depended on the
URL ending with a scalable image format suffix, which a data URL
doesn't. So e.g. an SVG in a data URL was always rendered at DPR 1.

For data URLs, map the media type to the image formats supporting it,
as declared by the image format plugins.

Fixes: QTBUG-120190
Pick-to: 6.12
Change-Id: I4e2bb42d4f6ce8474141f4e39ed974ccb97df3f5
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>