Transparency tryout.
Merge branch 'master' into Viktor

Conflicts:
	source/raytracer/raytracer.cpp
Merge branch 'master' into Viktor

Conflicts:
	source/raytracer/raytracer.cpp
Fix merge errors.
Merge branch 'master' into Viktor
Add rotation around arbitrary axis transformation.
Add a second path tracer implementation.
Some corrections in PathTracer2.