make starfield configurable, use setData instead of mapping

make the starfield configurable at runtime.
play around, try big values, small radius ...
the starfield can be deactivated by setting it's size to 0.

@psieg please test if it is better with the hd4000. I reduced the number of stars and implemented a faster buffer update. If it doesn't run good in release mode please active debugsymbols in linker->debug and send me a profiler run (the .vspx file). 
#347