JPEGFormat: seek(0) before checking lengths

When checking format compatibility of a RAIS, the JPEG signature is read
and then the entire stream is parsed to check the Metadata.

The initial signature read puts the stream off-index and causes the
subsequent read to fail. Thus, it now resets by seeking to 0 before
parsing.
JPEGFormat: seek(0) before returning from isFormat

Since the stream could still be used after JPEGFormat's isFormat check,
make sure to seek(0) before returning.

Also, the ImageIOFormat seemed to be closing the stream during parsing,
and thus during isFormat checking in JPEGFormat, which prevented seeking
again.

This close statement was removed to allow seeking and continued
checking.