Comment 4 for bug 1374393

Since the last comment, this has expanded into development of a new, modular postprocessing filter framework. Its development is tracked in this forum thread (containing also the code):

https://www.panda3d.org/forums/viewtopic.php?f=9&t=17232

The RGB<->HSL conversions mentioned above are (as of this writing) located in Desaturation.py.