Add support for MNG

[rayrobdod]

MNG has a very similar format to PNG - just with additional chunks and a different magic number.

• Check whether any MNG chunks have compressed parts
• Check whether the MNG chunks that do not have compressed parts are safe to copy, either marked as such or would be safe for this algorithm to copy
• Figure out how to check whether two MNG files have the same semantics

[rayrobdod]

Split off from #2