Fix: Can now drag double height toolbars onto start of last row, even when last row is single height.

I also simplified the code by (a) exiting from a loop as soon as we know the answer and by (b) not treating the initial case of a loop as 'special'.

git-svn-id: http://audacity.googlecode.com/svn/audacity-src/trunk@13475 47890b92-0858-11df-a26f-8b716316a5bc