pngdec: change parse logic

pngdec: change parse logic

Right now in parse logic the signature is checked every time the parse function
is called, and the whole data is the scanned each and every time, even though the
data is scanned in the previous instance. Changing the logic such that, we skip
the bytes which are already scanned in the previous instances of parse. This
helps in avoiding multiple scan of already scanned data/signature.

https://bugzilla.gnome.org/show_bug.cgi?id=737708