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

[CitizenOneX]

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).

[CitizenOneX]

I've made one correction, to scale the (max) normalized rgb value up to gain levels.
Previously the implementation had a set of gains 0..4 and a separate set of gains at 0..1023 for hardware level. To simplify the implementation I intended to scale all relevant values into this range but the normalized r, g, b values were missed.

Separately I've also initialized the "last" struct to start at a lower shutter value of 1600 again, it will ramp up if it needs to.