static areImageStreamsCompatible_(outputStream, candidate) {
    // For image, we don't care about MIME type.  We can always switch
    // between image types.

    return true;
  }
@param {T} outputStream A image output stream
@param {T} candidate A candidate stream to be combined with the output
@return {boolean} True if the candidate could be combined with the
  output

@template T
Accepts either a StreamDB or Stream type.

@private