[libpng16] Changed tabs to 3 spaces in png_debug macros and changed '"%s"m'

to '"%s" m' to improve portability among compilers.
Changed png_free_default() to free() in pngtest.c
[libpng16] Changed deprecated png_free_default() to free() in pngtest.c

Changed tabs to 3 spaces in png_debug macros and changed '"%s"m'
to '"%s" m' to improve portability among compilers.