Changed ProcessMosaic to no longer re-read a buffer in a tight loop for every...

Changed ProcessMosaic to no longer re-read a buffer in a tight loop for every pixel in an image when using the band priority mode.  Instead, an already read buffer is used and the desired pixel is accessed in constant time, removing memory leaks caused by tight reading and dramatically speeding up applications such as automos when using the PRIORITY=BAND option.  Consequently, the method GetPixelOrigin() was removed, as this was the only place it was currently being used.  These changes fix #346.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@3037 41f8697f-d340-4b68-9986-7bafba869bb8