For JPX decoding only read next packet after checking for byte aligning.

This patch makes the image from #5349 appear correctly, however artefacts
for the last packet seem to still be there.
This patch also optimizes some "in-checks" and adds a few header parsings.
Well yes :-) I only expanded the todo from the previous code. This was not handling colorspaces at all. Maybe I should reword this to warn: Todo ?