Port the lmmse gamma step to the other demosaicers

[kmilos]

s/gamma/variance stabilization transform/g and then it perhaps makes more sense:

Denoising: How variance stabilization transform work

While I was working on noise profile creation for darktable, I had to learn how variance stabilization transform work. So, here is a summary of what I learnt about this What are variance stabilization transform? Noise variance is not constant: the variance depends on the light intensity. Typically, variance is higher in bright areas than in dark areas. Plenty of noise algorithms are designed to work on images that have constant variance. To use these algorithms, we can trans…