NOTE

Currently the visual drag fails if the region is ragged, due to C-vjj$ on
something like the following string. 

1 1 1 1 1 1 1 1
1 2 1 1 1 1 1 1
1 3 1 1 1 1 1 1
1 4 1 1 1 1 1 1 1 1
1 1 1 1 7 1 1 1

A user can work around this limitation by doing a manual reselection after turning on
virtualedit so that they select a full rectangle. However it doesn't make sense
to require the user do that and it would be a better experience if we could
find the real right corner instead.

A more signficant bug / limitation is that selections starting from the
top-right corner do not work correctly.  It is likely that the easiest way to
fix this is to programmatically change the selection to be from the top-left
corner, but that might make the user experience a little awkward. It would be
good to preserve the original marks after re-selection. Thus, it likely makes
sense to track both the original coordinates and the top-left and bottom-right
corners. Use the original coordinates for re-selection, but use the top-right
and bottom-left for figuring out which row or column to copy where when
something is displaced.