The T.800 spec says there should only be one 'colr' box, but the
extended jpx file format spec in T.801 annex M allows having multiple.
Method 2 is a basic ICC profile, while method 3 (jpx-only) allows full
ICC profiles. Support that.
For the test, I opened buggie.png in Photoshop, converted it to
grayscale, and saved it as a JPEG2000, with "JP2 Compatible" checked
and "Include Transparency" unchecked. I also unchecked "Include
Metadata", and "Lossless". I left "Fast Mode" checked and the quality
at the default 50.