Support gradient calculations for RGB images about imageproc

[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.