demosaic tooltip: add VNG4

[esq4]

The method is used but there is no explanation.

[jenshannoschwalm]

good to me too.

[jenshannoschwalm]

Second thought about this:

1. VNG4 is a good option for snowfields and alike only for rare situations. Some camera/sensors seem to be more prone to those maze artefacts that can be observed using RCD but that's very rare.
2. When you are into those tooltips, VNG for xtrans is of clearly worse quality than all other demosaicers. For some reasons i don't understand there are strong artefacts at sharp yellow<->blue transitions. I am tempting to remove the VNG for xtrans ...