Add support for movie frame-rates based on elapsed solar time

[Keith Hughitt]

Currently, the movies generated by Helioviewer.org have a constant playback frame-rate. For example, 8 fps. This frame-rate does not take in to account, however, any variation in the cadence of the actual data-points. When a gap in the data is encountered, the movie simply "jumps" past it and continues playing.

In many cases, a more desirable approach would be to generate movies which have a constant playback rate with respect to the actual elapsed time, e.g. 10 seconds of movie time may represent 1 hour of elapse physical time.

It is possible to achieve this using the setpts video filter from FFmpeg's Libavfilter (http://www.ffmpeg.org/libavfilter.html).

See: http://ffmpeg-users.933282.n4.nabble.com/Is-it-possible-to-vary-frame-rate-through-out-a-movie-td2303626.html#a2305757.