This commit changes the `JpxImage.decode` method signature to define the
`ignoreColorSpace` argument as optional with a default value. Note that
we already set this default value in the `getBytes` method of the
`src/core/decode_stream.js` file since this option only seems useful for
certain special cases and therefore shouldn't be mandatory to provide.
Moreover, the JPX fuzzer is changed to use the new `JpxImage` API.