Cannot render with -o using multiple output filenames

rendering with husk with the -o argument only works for a single filename. If multiple filenames are appended (comma-separated) it's supposed to work with husk but this doesn't happen when the render products are in "arnold" type

Start by reproducing the husk render with -o using comma-separated filenames and render products of type "arnold". Trace the handling of multiple output filenames and verify that the render produces every requested file; done means the multi-filename case works for Arnold products as it already does for a single filename.

Written by the indexing model from the issue text.