libpng: upgrade 1.6.48 -> 1.6.50

libpng: upgrade 1.6.48 -> 1.6.50

Changelog:
Version 1.6.49 [June 12, 2025]
  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.

Version 1.6.50 [July 1, 2025]
  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`.

Ptests successfully passed:

============================================================================
Testsuite summary for libpng 1.6.50
============================================================================
# TOTAL: 32
# PASS:  32
# SKIP:  0
# XFAIL: 0
# FAIL:  0
# XPASS: 0
# ERROR: 0
============================================================================

Signed-off-by: Gyorgy Sarvari <skandigraun@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>