[cleanup] Get rid of ConnectionPoint::last_pos

Makes sizeof(ConnectionPoint)==32, before it was 48.
The single real user diagram_update_connections_object() was easy
to convert with other 'redundant' position information. So one less
extra field to maintain.
Fix (inlined) image related leaks

 - DiaImage::scaled was leaked
 - undo information of dia_object_set_pixbuf() was not freed completely
 - Standard - Image (_Image::pixbuf) was not properly reference counted