Fix bug in pair layout with a single column.

Code had assumed that (0, 0) would be the center of the layout
and represent "no offset", but this isn't the case.
Added test for repeat key in group symbolizer.

Test shows two items with different repeat keys ("foo" and "bar")
and rows of other group symbolizers sensitive to either one. The
reference images show that the "bar"-sensitive rows are displaced
by the "bar" item, but not the "foo" item.