Generate cLLI?

[ProgramMax]

cICP and cLLI are separate.
However, @svgeesus pointed out in a PNG meeting that it would be handy to have a tool like png_cicp_editor for cLLI.

It could auto-generate the cLLI by looking at the image contents and cICP.
(Open question: For APNG, should it ignore the static image if it is unused in the animation?)

Note: It could also warn when cLLI is provided for non-HDR color spaces.

There should probably be options to explicitly specify the values or allow them to be generated.
This could be a separate tool. But it would replicate so much of this tool in both functionality (add, overwrite, remove) and code (command line parsing, png parsing, file reading/writing) that it makes sense to add it to png_cicp_editor. At which point, perhaps the project should be renamed.

[ProgramMax]

Now that I think about it, same for mDCV and maybe even eXIf.

[ProgramMax]

If I make this a Third-Edition tool or a universal tool, the command line parameters should probably change to:
add cICP --preset display-p3
(ignoring case for "cICP", probably)
So more generally, the form:
action chunk parameters

[svgeesus]

(Open question: For APNG, should it ignore the static image if it is unused in the animation?)

This is well defined. From 11.3.2.8 cLLI Content Light Level Information

MaxFALL (Maximum Frame Average Light Level) uses a static metadata value to indicate the maximum value of the frame average light level (in cd/m2, also known as nits) of the entire playback sequence. MaxFALL is calculated by first averaging the decoded luminance values of all the pixels in each frame, and then using the value for the frame with the highest value.

and

Each frame is analyzed.

frame
For static PNG, the static image is considered to be the first (and only) frame. For animated PNG, each image that forms part of the frame-based animation sequence is a frame. Thus, for animated PNG, when the static image is not the first frame, the static image is not considered to be a frame.