[PNG] Safe-to-copy bit clarification

Hello everyone,

Previously, there was some question as to the meaning of the safe-to-copy
bit and what changes can be made by a PNG editor. I researched the topic
extensively. I have attached that research and its findings.

I am about to board a plane but when I land, I'll send a pull request to
rename mDCv, cLLi, and eXIf to mDCV, cLLI, and eXIF. My research leads me
to believe these chunks should be unsafe-to-copy.

An interesting note is the safe-to-copy bit really means there is a
dependency on data outside the chunk. I explain it in the research.

Have a good day!
-Chris B