Use I;16 mode for 9-bit JPEG 2000 images

[scaramallion]

Fixes #7899 .

Changes proposed in this pull request:

• Fixes the mode setting for 9-bit J2K images
• Adds a 9-bit test image

Reference: ISO 15444-1, Table A-11

[scaramallion]

Now that I look at it again, why is it using yrsiz[0] to determine the mode? yrsiz is the vertical separation of the component with respect to the reference grid. I would've though it'd be (ssiz[0] & 0x7F) + 1 for signed images and just ssiz[0] + 1 for unsigned.

[scaramallion]

Ah, there's another bug further up in parsing the header that's causing the ssiz value to end up in yrsiz, this should be 38 + 3 * i:

for i in range(csiz):
        ssiz[i], xrsiz[i], yrsiz[i] = struct.unpack_from(">BBB", siz, 36 + 3 * i)

[scaramallion]

Should I fix that as well, or should it be a separate issue?

[radarhere]

I think either should be fine.
If it's something producing a problem that you can test for, I'd say create a new PR with a test. Otherwise, if it's just a cleanup to make sure things are named correctly, here is good.