Ensure replaced tiles are repainted

If a layer with a backing store is resized, we may replace some tiles
that were not at maximum size. When that happens we need to ensure the
replaced area is repainted, otherwise the content disappears until
refreshed.

Change-Id: Ie2f7796a2493a56ed5fc6cc4cea55f4ab9f38bbe
Reviewed-by: Michael Bruning <michael.bruning@digia.com>