ensure_min_length_buffer!

[kornelski]

Improvement over #196

More info: image-rs/image-png#534 (comment)

[fintelia]

Sorry, I've mean meaning to write up a reply for this.

I think this is an improvement over the gif crate's current byte-at-a-time state machine loop. It reduces the number of states and makes the decoding portion clearer. I'm not a big fan of using macros for control flow, but uh, that's what goto! is already doing so I think it is fine having ensure_min_length_buffer! do the same.

As far as the broader question of how to structure decoders, I'm less convinced that a single forward pass state machine is necessarily best. There's a bunch of different options with different tradeoffs. Some of the simple decoders in image directly use read_exact to pull data they need. The decoder in image-webp scans through the file at the start to discover metadata and the locations of chunks and then later seeks back when it needs to read those chunks, which seems to work pretty well there. And there's also the possibility of hybrid schemes. libpng decodes most chunks using a high level state machine, but handles all the (consecutive) IDAT chunks within a single function.