Destringtypify the WebP container decoder. Fix unsupported contained formats returning DecodingErrors. Read fixed-sized chunks into stack buffers instead of Vecs. Fix potentially truncating chunk type. Reorder signature checks
Destringtypify the WebP container decoder. Fix unsupported contained formats returning DecodingErrors. Read fixed-sized chunks into stack buffers instead of Vecs. Fix potentially truncating chunk type. Reorder signature checks

The take(N).read_to_end() chain could terminate before reading N bytes,
(if, e.g., the reader returned Ok(0) only temporarily) therefore
truncating the chunk type, and most likely sink()ing a good bit of the
data

Ref: #1195

PistonDevelopers/image,PistonDevelopers/image