Fill Layers: re-render the whole layer if the image bounds change

Before this commit, if a Fill Layer was to be re-rendered as a
consequence of an image resize, it would improperly attempt to preserve
the existing rendered quadrant.

To fix this, the image bounds are saved after an update job is called.
If they change in the next update, the preparedRect is cleared and the
layer is completely re-rendered again.

CC: kimageshop@kde.org