Give gain slider more flexibility

[calcmogul]

There's currently one "gain" slider that ties the red and blue channels together, but low gain/exposure settings results in green images under fluorescent lighting. This is a problem if the target is lit green; the user has to use a much wider hue passband.

One solution to this would be separate red, green, and blue exposure sliders. A button could be added to autoselect the settings for them when the targeting LEDs are off (aka "auto white balance").

MATLAB's docs mention a few different algorithms that I imagine would be a starting point.
https://www.mathworks.com/help/images/comparison-of-auto-white-balance-algorithms.html

OpenCV has this, but idk if it's helpful: https://docs.opencv.org/4.5.2/d7/d71/classcv_1_1xphoto_1_1GrayworldWB.html

Another option is turning on auto white balance of the camera temporarily to get the values, then turn it back off.

[mcm001]

Per discussion on Discord:
We can expose the red and blue balances (raspicamcontrol_set_awb_gains), as well as expose contrast/saturation. We have control over the digital gain, but not analo. Those might best go under an advanced dropdown or something, as it's starting to get cluttered.

[mcm001]

The gain slider was previously broken; latest version shouldn't have this issue. Do we still think it's valuable to split the gain slider?

[gerth2]

After playing with it myself .. I vote no, single slider works plenty well.

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