Parallelised luminance conversion routine and also converted floating po...

...int to integer math. Difference in result from FP math is on average less than 1 level. Decent performance boost =)

This form might be more amenable to vector processing, but I didn't attempt to; merging the rotation code with the acquisition code is *the* next low hanging fruit for performance gains.

For the time being the introduced/changed variables are suffixed with double underscores for easy reviewing, but let me know the preferred style and I'll adapt accordingly.