Provide better seeking and position display for some videos.

Specifically, there are examples of poorly-encoded avi files that
report a video frame rate of 30fps but the video stream is actually
24fps.  Because frame intervals are actually encoded as 30fps, every
4th frame has to be repeated.  Without an adjustment to framesPlayed,
translations between MythPlayer's frame-based seeking/display and
ffmpeg's timecode-based seeking drift out of sync.

Note: Similar adjustments may be necessary if frames have to be
dropped due to a frame rate mismatch.

Refs #11415.