Determine Sync metadata for samples from libavformat / libavcodec without decoding

[vade]

It seems like our Media Format Extension is super close, but we need to understand how to provide the MESampleCursor implementation with some metadata about key frame, sync and dependencies

• [x] AVSampleCursorSyncInfo
• [x] AVSampleCursorDependencyInfo
• [ ] MEHEVCDependencyInfo
• [ ] decodeTimeOfLastSampleReachableByForwardSteppingThatIsAlreadyLoadedByByteSource
• [ ] samplesWithEarlierDTSsMayHaveLaterPTSsThanCursor
• [ ] samplesWithLaterDTSsMayHaveEarlierPTSsThanCursor

[vade]

We may also need to implement all of the cursor delivery methods too - eep

See MESampleCursors Obj-C header for inline docs