package/libpng: bump version to 1.6.29

Refactored patch 0002, added support for Intel SSE2.

Changelog:
https://sourceforge.net/p/libpng/code/ci/b363e01e6b5e865e536db62ca4480f3a2d4690bd/#diff-2



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>