feMerge - extract the list of inputs to merge before doing the merging
feMerge - extract the list of inputs to merge before doing the merging

For filter primitives that require parameters from their nodes'
children, we'll first build that list of parameters by walking the
children or whatever, and then doing the final processing on nice data.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg