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.
this makes sense to me. I will change the code to 

    alignToByte();
    var packet = packetsIterator.nextPacket();

and see what regression tests think of it.