*** NOT FOR LANDING *** mapLocalToContainer and offsetFromContainer cleanup.

*** NOT FOR LANDING *** mapLocalToContainer and offsetFromContainer cleanup.

The ultimate goal is to get rid of that multicol kludge in
LayoutBoxModelObject::mapAbsoluteToLocalPoint().

After I managed to untangle multicol and other stuff from offsetFromContainer(),
it became easy to split it into proper sub-class implementations again
(that's patch set 7 and 8).
I'm not sure if that's desirable, though. We'll get more function calls than
before this way, and, after all, most objects in the containing block chain are
LayoutBox-derived, so we'll have a lot of LayoutBox::offsetFromContainer() calling LayoutBoxModelObject::offsetFromContainer() calling LayoutObject::offsetFromContainer().