qml: use `paintedWidth` and `paintedHeight` first in DoubleShadow
Unless clipping is enabled, paintedWidth
and paintedHeight
provide more information on the actual region that the source
item rendered content.
These properties are provided in QQuickImage
. However, it is
not necessary that the type is QQuickImage
. We should respect
each type that provides such information.
Request review @chub.