Add support for encrypted MP4 sample groups

[theodab]

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Currently, we only support MP4 encryption when not using sample groups. This is the most common case, but when using key rotation, samples encrypted with different keys will use different sample groups. We'll need to update FFmpeg to handle this. FFmpeg already supports sample groups, but we need to add support for the seig sample group entry that contains the group-specific encryption info. Then we need to associate each frame with the sample group so we can determine which encryption info to use.