Play FMVs using ffmpeg instead of smackw32.dll

[xtremeqg]

This PR will play videos using ffmpeg instead of relying on smackw32.dll. This a required change for cross-platform support.

Because there's no OpenAL yet, I've added (temporary) audio playback through SDL. There's some occasional glitching, likely due to buffer underruns. This can be solved by buffering a few frames in advance.

The size of ffmpeg can be reduced by playing with configure options, it currently includes a lot of codecs and other formats which are not used.