Support gradient calculations for RGB images

[theotherphil]

See https://mht.technology/post/content-aware-resize/

filter3x3 already supports this, so we'd just need to decide how to combine the per-channel magnitudes. It's not obvious what the correct approach is here.

[theotherphil]

Some notes on approaches to defining gradients for colour images: http://ai.stanford.edu/~ruzon/compass/color.html

I think we shouldn't bother trying to do anything clever. Instead, we should just provide a method which computes per-channel gradients and applies a user-provided function to combine them, with two standard options provided:

CombineChannelGradients {
Max, // Use max of per-channel gradients
Mean, // Use mean of per-channel gradients
}

From some quick googling these both appear to be reasonably common.