Fix for background margins

First part of a fix for #874. This PR removes our delegation to the parent
margin for users of ParentDimension (which is only background and border). Both
of these cases already have the margin factored in, so we were doubling up on
it.

I also found in the course of this that the check for needing positioning
didn't seem to matter given how the parent dimension lookups override values
for us in the background and border. Less code == more better, so removed.

I'm going to run through some samples on this to confirm nothing's too weird.
Note that there's another PR inbound after this that will deal with the
backgrounds running past the end of the containing parent.