Lazy parse sBIT

[kornelski]

Alternative to 2949036 (#554)

Needs image-rs/fdeflate#43

I like the previous solution with warnings: Vec<Error>, since it's easy to implement, uniformly handles all ancillary chunks, keeps Info small, and is lossless.

Here's an alternative solution that keeps one parse error per chunk.

In either case, I think it's better to keep the errors than to discard them. It seems weird to have all these parsers and error enums that just get discarded. That's dead code.