make starfield configurable, use setData instead of mapping

make the starfield configurable to runtime.
play around, try big values, small radius ...

@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)