pngrtran.c to suppress clang warnings (Bug report by Viktor Szakats).

  Removed some useless typecasts from contrib/tools/png-fix-itxt.c

(Bug report by Viktor Szaka'ts).

the Coverity scan without them.

Work around more Coverity-scan dead-code warnings.

    needed by png_reciprocal2().
Added #ifdef PNG_16BIT_SUPPORTED/#endif around png_log16bit() and
    png_do_swap().

"if (x !== 0)"

internal functions called by png_do_{read|write}_transformations static. On an
x86-64 DLL build (Gentoo Linux) this reduces the size of the text
segment of the DLL by 1208 bytes, about 0.6%. It also simplifies
maintenance by removing the declarations from pngpriv.h and allowing
easier changes to the internal interfaces.

function definitions before the place where they are called so that
they can be masde static. Move the intrapixel functions and the
grayscale palette builder out of the png?tran.c files. The latter
isn't a transform function and is no longer used internally, and the
former MNG specific functions are better placed in pngread/pngwrite.c

pngrtran.c