Auto exposure and white balance loop: scale per-channel rgb gains so none exceed new parameter rgb_gain_limit

Per-channel (r, g, b) gains seem to contribute most to the row noise that presents in (rotated) images as vertical lines. Rather than use the full range of r, g, b gain (0..1023), allow users of the auto exposure and white balance algorithm to set a cap on the rgb gain, and set a default on the low end of that range (141).