public abstract class Track
A class representing a track contained in a media resource.
A media resource may have multiple parallel tracks, such as a video
track with several audio tracks in different languages. The types of tracks
supported by the system may be inferred from the existing subclasses of this
class. Not all media resources will contain a track of each supported type,
and the time span of a given track need not be commensurate with the time
span of the containing media.