Fix various issues with the vc9 solution and assimp_cmd.
Declare some more Importer methods const. This marks them as safe to use from within a progress callback.
git-svn-id: https://assimp.svn.sourceforge.net/svnroot/assimp/trunk@806 67173fc5-114c-0410-ac8e-9d2fd5bffc1f