The track identifier from an instruction source frame.
- iOS 7.0+
- macOS 10.9+
- Mac Catalyst 13.0+
- tvOS 9.0+
If the video composition result is one of the source frames for the duration of the instruction, this property returns the corresponding track ID. The compositor won't be run for the duration of the instruction and the proper source frame will be used instead. The value of this property is computed from the layer instructions