#64 Consider adding G'mic or other advanced raster effect processor

Can be implemented after #40 is done. Still, can be quite hard to implement properly (i.e. to make it possible to apply it to any object individually).

make it possible to apply it to any object individually

This can be implemented via ExternalRasterProcessor node, which would render its children (possibly with transformations applied, with configurable resolution) and then process its output in external processor.