Merge branch 'MA_Pape_2018' of...

Merge branch 'MA_Pape_2018' of https://devhub.vr.rwth-aachen.de/VR-Group/Project_Phoenix into MA_Pape_2018 and revoked the sharing of acceleration structures due to wrongly rendered lenses.

# Conflicts:
#	demos/optical_bench/src/lens.cpp
#	demos/optical_bench/src/lens.hpp
#	demos/optical_bench/src/optix_context_manager.cpp
#	demos/optical_bench/src/optix_context_manager.hpp
#	demos/optical_bench/src/ray_pass.cpp
#	demos/optical_bench/src/test_pattern_frame.cpp