Various fixes needed for ensuring that the molecule viewer keeps running well on low-spec / software-only OpenGL implementations.
- disable eyecandy in "Low quality" mode
- by the way, always disable the zoom's eyecandy, it's ugly.
- only enable the second light in "High quality" mode
- only keep asserts in libavogadro in 'debugfull' mode.

This prompted changes to libavogadro which I'm posting upstream.