Support decoding into preallocated memory

[Ralith]

This is useful for skipping a potentially large copy when e.g. uploading texture data to a GPU, by decoding directly into a driver-allocated staging buffer or mapped memory. It would make sense for higher-level, more magical owned-memory decoders to be defined in terms of such a primitive. Compare the png crate's next_frame method.

[Ralith]

It would also be nice to support encoding into preallocated memory, or even arbitrary io::Write impls.

[kornelski]

I've looked at io::Write before, but it won't work with the current implementation which writes things out of order (e.g. needs to re-read its own data to set checksums). Also the library can reserve memory it needs, but io::Write to a Vec would need to keep resizing the buffer.

However, decoding to a pre-allocated buffer makes sense and should be easy to imeplement.

[Ralith]

it won't work with the current implementation which writes things out of order (e.g. needs to re-read its own data to set checksums).

If computing these checksums on the fly is too onerous, io::Write + io::Seek could be used. An in-memory convenience function could still query the total required storage and reserve space in a Vec (or on the filesystem, or ...).

Decoding's the case I'm actually affected by, though.