After getting the I/O between the AVR and the Dragon's Lair working properly yesterday, I turned my attention to getting video playing on the PC. I was seeing my CPU usage pegged at 100% and no video which was disconcerting since I knew this PC was fast enough to play the video while running real-time linux (which is slower than normal linux).
I found two things. The first one was that my code that automatically drops fields if the PC is behind was not working, so it was getting more and more behind. The other thing was that somehow I had switched to using the slowest type of JPEG IDCT algorithm. Once I switched it back to my hand-crafted SSE2 algorithm (which ended up not being much faster than the reference integer version unfortunately) things improved dramatically and the PC was then able to play video at full speed using about 75% CPU which is acceptable (considering it doesn't need the CPU for any other purpose).
So I made a video of the attract mode playing with the video being rendered on the VGA monitor. I did attempt to route through the arcade monitor on the Dragon's Lair but my usual electrical problems made this underwhelming. I've included it in the video anyway.
Sound isn't working but I haven't spent any time looking into why. It's probably something simple.