Improve rectangle direction generation

The previous solution didn't take into account that the source rectangle
might be at either end of the selection, thus giving it the wrong
direction of CENTER, while it should have been RIGHT or LEFT.

Test: manual - verify that the rectangles expand properly in a smart
select animation when there are multiple rectangles and the source is
not in the center - e.g. a multiline case with the selection starting on
the first line

Change-Id: Id18ab8f9918e74a576503620dae2f7cdd593fb0e