overhaul GXFIFO packed-command handling to more closely resemble a reasonable hardware behaviour. this is unlikely to have fixed anything, since previous revisions had somehow smashed their way through most of the bugs; however, the previous revision did have a tiny potential bug which might have been fixed by this (array out of bounds access). the code is so pretty now, I think it must be right, but there's a chance that I have created a new bug.

git-svn-id: https://svn.code.sf.net/p/desmume/code/trunk@5031 5577840e-afb3-4e59-9ef3-2b7f898b1952