mj2_video_source::get_frame_period

Overrides kdu_compressed_video_source::get_frame_period

If no image is currently open, this function returns 0. Otherwise, it returns the number of ticks associated with the frame to which the currently open image belongs. The number of ticks per second is identified by the get_timescale function. If the video is interlaced, there are two images (fields) in each frame period.