Invalidate the entire sheet after having inserted the image. cp-5.1-14

This is a hack; but the ViewObjectContacts are unreliable with tiled
rendering, and we are missing invalidates due to that.

Change-Id: Ia2475d4a042be1db0ea6f170cf66f271397ee299
Reviewed-on: https://gerrit.libreoffice.org/31659
Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>