Added conversions from gray8 to rgba16/32.

git-svn-id: https://agg.svn.sourceforge.net/svnroot/agg@60 ab9de31f-a152-4fe9-8809-95f286f9ca89
Removed useless conversions.

git-svn-id: https://agg.svn.sourceforge.net/svnroot/agg@61 ab9de31f-a152-4fe9-8809-95f286f9ca89