[r5031] overhaul GFXFIFO packed-command handling tomore closely resemble a
reasonable hardware behaviour. This is unlikely to have fixed anything, since
previous revisions had somehow smashed their way through most of thebugs; however, the prvious revision did have a potentail 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'sa chance that I have created a new bug