[feature] New filters for CommonFilters

[Juha Jeronen]

New filters in this patch:

- Lens flare based on the algorithm by John Chapman. Panda implementation originally by ninth on the forums. I've just cleaned up the code a bit and made it to work together with CommonFilters.

- Desaturation (monochrome) filter with perceptual weighting, and configurable strength, tinting and hue bandpass. Original.

- Scanlines (CRT TV) filter (original). Both static and dynamic modes, configurable strength and which field to process, and the thickness of the desired scanlines (in pixels). Original.

Forum topic acting as a development blog:

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

N.B.! This patch depends on first applying the cartoon shader patch in:

https://bugs.launchpad.net/bugs/1221546

[Juha Jeronen (juha-jeronen)]

Example programs showing how to use the lens flare. Contains binaries (textures), so included as a zip file.

This should go into samples/.

Contains a "Basic" tutorial in the same format as the existing ones, and ninth's original code as an "Advanced" tutorial.

[Juha Jeronen (juha-jeronen)]

Oh, and before I completely forget:

On the forum, a couple of years back, someone wanted RGB<->HSL colour conversions into Panda's library:

https://www.panda3d.org/forums/viewtopic.php?t=8877

This patch provides my own version of such conversions (in Python; also based on information from Wikipedia), but only as local to CommonFilters. These could be moved into some sensible place in the hierarchy.

[Juha Jeronen (juha-jeronen)]

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.