Helioviewer.org Movies should not include redundant frames

To improve speed and reduce file-size for movies generated by Helioviewer.org, redundant movie frames or image layers should be avoid.

There are two cases to address:

1) If all image layers for a given time-step are the same, then that time-step should be dropped.
2) If one or more of the image layers for a given time-step are the same, then for each image that is used in other frames, that image should be generated only once a re-used in each other composite image/movie frame as needed.