libpng: Update to version 1.6.50

libpng: Update to version 1.6.50

- Update from version 1.6.48 to 1.6.50
- Update of rootfile
- Changelog
    1.6.50
	  Improved the detection of the RVV Extension on the RISC-V platform.
	    (Contributed by Filip Wasil)
	  Replaced inline ASM with C intrinsics in the RVV code.
	    (Contributed by Filip Wasil)
	  Fixed a decoder defect in which unknown chunks trailing IDAT, set
	    to go through the unknown chunk handler, incorrectly triggered
	    out-of-place IEND errors.
	    (Contributed by John Bowler)
	  Fixed the CMake file for cross-platform builds that require `libm`.
    1.6.49
	  Added SIMD-optimized code for the RISC-V Vector Extension (RVV).
	    (Contributed by Manfred Schlaegl, Dragos Tiselice and Filip Wasil)
	  Added various fixes and improvements to the build scripts and to
	    the sample code.

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