camera: SoftHDR: Reinhard tonemap and parallel processing

On an idea from Steve Kondik, the RS script now uses
parallel processing via rsForEach, processing every
line of the image in multiple threads.

I updated the tone mapping process as well, using
Reinhard's algorithm rather than a simple average.
This results in much more colors in the images, rather
than grayish/dull ones you might get previously.

Change-Id: I552c61e10abf652c50b8511169886bab0986637e