In ProcessMapMosaic, StartProcess() was modified to account for negative x or...

In ProcessMapMosaic, StartProcess() was modified to account for negative x or y values from the projection. These x and y values are used to calculate outline and outsamle. Fixes #1736.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@5588 41f8697f-d340-4b68-9986-7bafba869bb8