Upgrade libpng to version 1.6.16
升级libpng到v1.6.16

Signed-off-by: Wanpeng Song <swpustc@mail.ustc.edu.cn>
Merge branch 'sources' into projects

Signed-off-by: Wanpeng Song <swpustc@mail.ustc.edu.cn>
preprocess libpng sources
预处理libpng源文件

Signed-off-by: Wanpeng Song <swpustc@mail.ustc.edu.cn>