Apply size constraints for direct compositing also to background images.

The fact that this was not done before led to a failed WebKit assertion
in the UIProcess which caused any WebView application loading pages
with large background images (e.g. flickr.com) to crash on Windows.

Task-number: QTBUG-35208
Change-Id: Ieae76e19fc08483cbd038a7c14e06b4bd12dd681
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>