Removed "master alpha" feature from compound rasterizer, as it doesn't work (and can't be made to work).
Changed mutating methods in color types to return non-const references, in order to allow method chaning.

git-svn-id: https://agg.svn.sourceforge.net/svnroot/agg@74 ab9de31f-a152-4fe9-8809-95f286f9ca89