NEW FETURE: Allow reset of just a single colour channel on the live stack

one thing that might be nice for a future Starlight Live version is the ability to clear the data from one channel only from a stacked image, specifically for narrowband multispectral imaging. I've had a couple of times where one particular channel gets bad images (focus issues or drift) and have wanted to be able to clear that channel only, instead of resetting the whole image.