loaders: PNG: Change the ReadPNG() and OpenPNG
loaders: PNG: Change the ReadPNG() and OpenPNG

The ReadPNG() now expects the PNG header too and the OpenPNG() does
rewind the file after the signature is checked.

Signed-off-by: Cyril Hrubis <b1e90efd3b808b8e0bcf2a9a55fd8b9a779bfd81@ucw.cz>

gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim