Change Decoder to a BufRead API

[bushrat011899]

Objective

Decoder currently wraps any provided reader R in std::io::BufReader, as the decoding process is designed around buffered reads using std::io::BufRead. This can lead to double-buffering issues if the provided reader R already implements std::io::BufRead on its own, leading to possibly lost performance.

Solution

• Adjusted Decoder<R: Read> into Decoder<R: BufRead>. This is a breaking change users may need to accommodate by replacing Decoder<...> with Decoder<std::io::BufReader<...>> in type signatures.
• Changed DecodeOptions::read_info to accept an R: BufRead. For migration, you may need to wrap the R parameter in std::io::BufReader::new(...).
• Changed Decoder::new to accept an R: BufRead. For migration, you may need to wrap the R parameter in std::io::BufReader::new(...).

[bushrat011899]

See this comment regarding the CI failure.

[bushrat011899]

See #206 for CI failure cause and possible fix.

[fintelia]

I think this is reasonable. The image crate already requires the BufRead bound to decode GIFs in preparation for the possibility that we'd want to make this change.

(There was a slight oversight where GifDecoder::new doesn't require the bound, but calling any methods on the resulting decoder does. Technically that means it would be a breaking change to switch it, but given the limited expected impact, it might be fine to do without a major version bump)

[kornelski]

Related discussion:

image-rs/image#2472