rewrote stereo code to work with the new shader post-processing concept

re-arranged some of the scene rendering code in general
some more work on the broken shadow mapping, not yet complete
various small tweaks


git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/branches/new_gfx@2193 5a81b35b-ba03-0410-adc8-b2c5c5119f08