[master] Revised png_get_uint_32, png_get_int_32, png_get_uint_16 (Cosmin)

Moved reading of file signature into png_read_sig (Cosmin)
Fixed atomicity of chunk header serialization (Cosmin)
Added test for io_state in pngtest.c (Cosmin)
Added "#!/bin/sh" at the top of contrib/pngminim/*/gather.sh scripts.
11 files changed