Speedup

Hi again, this one is more of a starting point for discussion: 

I've done some profiling, and it looked like decodePixelMeasurement was consuming a significant amount of CPU time (~ 15%). To optimize that, I created a loop-based version from the original libfreenect1 code which unpacks all 10 subframes at once instead of pixel-by-pixel. However, the resulting speedup appears to be around ~ 2% at best, so it's not even clear if there is any real effect at all or just measurement noise...

Thoughts?

Florian