Implement gamma optimization option from TruePNG

[shssoichiro]

https://css-ig.net/articles/truepng.php

in most of cases, it is better to disable any chunk that correct the way the image is displayed, specially for Web. gAMA chunk is not an exception, and the -g0 should delete it. However, TruePNG let you to merge the way the image is displayed instead of using gAMA chunk.

be warned, it is a no back process, so you should make a backup of your file (or use -out switch). gAMA chunk can be added or deleted, but in this case, it changes RGB values in the new file.

• [ ] Implement gamma merging
• [ ] Research other headers this could be used for

[kornelski]

The correct solution for all of gAMA, cHRM, and iCCP is to convert to sRGB using color profile transformation. If you're OK having dependency on LCMS2, I can supply required code.

[shssoichiro]

Thanks! I don't mind adding that dependency.