Compute metadata scene related information

[quietvoid]

Samsung's HDR10+ tools expect scene information to be present for a valid JSON file.

Should be present for every metadata frame in SceneInfo:

SceneFrameIndex: Index of the frame relative to the current scene (starting from metadata change)
SceneId: Incremented scene index (Id) when metadata changes
SequenceFrameIndex: Metadata frame index (total length should be number of frames)

Should be present at the JSON root:

"SceneInfoSummary": {
    "SceneFirstFrameIndex": Array of `SequenceFrameIndex` where a metadata's `SceneFrameIndex` is 0
    "SceneFrameNumbers": Array of the number of frames (length) of every scene
}