Let alt be a float for benefit of D108 and D109 protocols to make more

Let alt be a float for benefit of D108 and D109 protocols to make more
immunune from rounding resulting from integer truncation.
(Why do I sense 'm going to regret doing this so late in the release cycle?)

git-svn-id: http://gpsbabel.googlecode.com/svn/trunk@954 f51c46e8-681c-474f-0cfe-069cfd0219fb