HIPS: convert to png on the fly?

When producing HIPS images, it would be useful to convert to png (using some specified color transfer function) on the fly.

This is both a common use case for me, which helps with compression, and basically a necessity for FITS cubes because there is no such thing as a .png cube.

The issue does not identify files, tests, or an entry point. Start by locating HIPS output handling and the existing FITS/image conversion paths, then determine how a color transfer function and on-the-fly PNG output should be configured. Done means HIPS images can be converted to PNG, including FITS cubes, with compression benefits and documented behavior.

Written by the indexing model from the issue text.