Add hiDPI @2x retina image support to AnimatedImage

Copied hiDPI image support from base Image-class in to overwritten load()
function that ignored it before.

Change-Id: Iee89b572b523bbf990b6dd0e94c59c71f2b2ed31
Task-number: QTBUG-43104
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>