Document png_set_background final argument and PNG_BACKGROUND_GAMMA_UNIQUE

[benkasminbullock]

The final argument of png_set_background doesn't appear to be documented in libpng-manual.txt:

http://www.libpng.org/pub/png/libpng-manual.txt

or in other online pages such as this:

https://refspecs.linuxbase.org/LSB_3.1.0/LSB-Desktop-generic/LSB-Desktop-generic/libpng12.png.set.background.1.html

Looking at the source code, the value is put into png_ptr->background_gamma on line 157 of pngrtran.c. The only time this appears to be used though is if the value of background_gamma_code is set to the undocumented value PNG_BACKGROUND_GAMMA_UNIQUE.

Shouldn't this all be documented, especially that the final argument is basically ignored unless the _UNIQUE value is chosen?