Match against all 8 PNG file signature bytes

[sudhir-b]

Update the PNG matcher to match all 8 bytes of the PNG signature as in: https://www.w3.org/TR/png/#5PNG-file-signature

I'm not sure that this makes any functional difference but spotted a discrepancy and thought I'd open this PR for completeness.

As an aside, it looks like the APNG matcher matches against an identical bytearray to begin with - is there a reason for this difference in the two matchers?

[CatKasha]

APNG matcher was made by me and i forgot(?) to edit PNG matcher Feel free to edit APNG matcher to use if instead of bytearray

[sudhir-b]

Done, thanks for looking - is anything else I need to do?

[CatKasha]

Looks good to me