Support in-place decoding

[fintelia]

Currently, decoding an image allocates a new vector to store the output data. This is unfortunate because it can result in extra copies, for instance in the image crate:

https://github.com/image-rs/image/blob/6e0cd31a5287dd589d2e78ae33c1f720c77a6863/src/codecs/tiff.rs#L198

It would be better if there was an API that took a mutable slice and wrote image data into that.