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.
ack, that is what i thought, but it is effectively a noop when invoked multiple times (skipNextBit never was set to true for me)