[Image Wallpaper] Fix blur

This broke in the transition to StackView as the blur needs a full-size source. Additionally

• put the item into a Loader to avoid needless shader compilation and other work
• only load the blur for image fill modes where the image does not cover the full size
• don't render background color when blur is enabled as you cannot see it anyway

Differential Revision: https://phabricator.kde.org/D11981