camera: SoftHDR: Use 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.

Change-Id: I552c61e10abf652c50b8511169886bab0986637e
Merge remote-tracking branch 'origin/cm-10.1' into cm-10.1-linaro