Add back support for sword1/2 MPEG-2 cutscenes

This adds a way to play the sword1/2 MPEG-2 cutscenes again, relying on libmpeg2 for the frame decoding. libmpeg2 is still an optional library.

There are many changes from the original mpeg2 code in ScummVM to make it not terrible. First, the file is actually parsed as the AVI file it is instead of just ignoring the container. Secondly, the overlay is no longer (ab)used to display the video and is displayed as long the backend supports a >8bpp screen.

The main reason for returning this is because the Sold-Out releases of the games were bundled with the MPEG-2 versions. As you can see, it's not a lot of code, so the impact is pretty minimal. I have initial playback for the RTZ videos in my other branch, but that's far off from being usable and I wanted to get this in the master to keep it from bitrotting.