Added advice about setting pointers to NULL after png_free(),
capitalized sentences, reformatted long comments in libpng style.

(from the git master branch)

Should refer to "pngwin.def" instead of "pngw32.def"

(should refer to pngwin.def not pngw32.def)

There are too many master directories floating around now...

It is no longer used since the MMX support was removed.

This avoids a "shadowed declaration" variable when compiling with
"gcc -Wshadow ..." due to another use of this name in math.h on
some platforms.

This avoids a "shadowed declaration" warning from "gcc -Wshadow"
due to math.h using "gamma" on some platforms.