Improve chunk parser flexibility when decoding incomplete input data.

[jangko]

Currently nimPNG parser and decoder works in 'strict' mode. It will reject any invalid input. But there are some classes of invalid input that can be just ignored or skipped. This 'relaxed' decoding mode of course will probably produce incomplete image, but in certain situation this is acceptable or even desired.

In the future perhaps we can add more relaxation. Right now we can focus to implement invalid chunk skipper.
This relaxed mode and any other future relaxation should be accessible via decoder flags/settings/configuration and not forced to user. User should be able to select how much data or which recovery algorithm they wanted. And user should be able to stick with 'strict' parsing mode if there is no room for corrupted/incomplete input.

see also issue #55.