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.
Ok, i make a combined case fall through section for all not yet implemented headers. I added the cases during debugging and thing they are useful for later extension/debugging, thats why i left them in without any real code.