prichunkpng does not adhere to chunk ordering

[drj11]

Consider the PNG chunk ordering: https://www.w3.org/TR/2003/REC-PNG-20031110/#5ChunkOrdering

When there is a PLTE chunk, some chunks (iCCP, sRGB, sBIT, gAMA, cHRM) must precede PLTE, some chunks (tRNS, hIST, bKGD) mus follow PLTE.

So when adding chunks to a PNG file some ordering constraints must be enforced.

Example: Adding PLTE to a PNG file with a bKGD chunk should put the PLTE chunk before the bKGD chunk. Currently the tools puts it after.

Example: Adding PLTE to a PNG file that has both sBIT and bKGD chunks should possibly re-order sBIT and bKGD so that sBIT precedes bKGD and PLTE ends up between them.