Address thread safety with Decode objects

[melissalinkert]

See ome/bioformats#3505.

First step is a multi-threaded unit test that reads existing test files and makes sure the decoded pixels are consistent.

What we need in the end is some API that can decode image streams in multiple threads, and either internally handle or report pixel type/BGR-ness in the process. If fixing new Decode(byte[]).toBytes(ByteBuffer) is feasible, that's probably easiest. If we have to write something new, that's OK too.