libpng: Update to version 1.6.48

libpng: Update to version 1.6.48

- Update from version 1.6.45 to 1.6.48
- Update of rootfile
- Changelog
    1.6.48
	  Fixed the floating-point version of the mDCv setter `png_set_mDCv`.
	    (Reported by Mohit Bakshi; fixed by John Bowler)
	  Added #error directives to discourage the inclusion of private
	    libpng implementation header files in PNG-supporting applications.
	  Added the CMake build option `PNG_LIBCONF_HEADER`, to be used as an
	    alternative to `DFA_XTRA`.
	  Removed the Travis CI configuration files, with heartfelt thanks for
	    their generous support of our project over the past five years!
    1.6.47
	  Modified the behaviour of colorspace chunks in order to adhere
	    to the new precedence rules formulated in the latest draft of
	    the PNG Specification.
	    (Contributed by John Bowler)
	  Fixed a latent bug in `png_write_iCCP`.
	    This would have been a read-beyond-end-of-malloc vulnerability,
	    introduced early in the libpng-1.6.0 development, yet (fortunately!)
	    it was inaccessible before the above-mentioned modification of the
	    colorspace precedence rules, due to pre-existing colorspace checks.
	    (Reported by Bob Friesenhahn; fixed by John Bowler)
    1.6.46
	  Added support for the mDCV and cLLI chunks.
	    (Contributed by John Bowler)
	  Fixed a build issue affecting C89 compilers.
	    This was a regression introduced in libpng-1.6.45.
	    (Contributed by John Bowler)
	  Added makefile.c89, specifically for testing C89 compilers.
	  Cleaned up contrib/pngminus: corrected an old typo, removed an old
	    workaround, and updated the CMake file.

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>