frame-margin property needs to be changed to temporal-margin #531

With the upcoming VFR changes in #168 we need to avoid using frame-based properties. One that comes up is frame-margin for the save-image command. We should phase this out and replace it with temporal-margin that functions the same, but using time instead of frames (e.g. temporal-margin = 0.1s).

Frame margin is also documented as:

Number of frames to pad each scene around the and end (e.g. moves the first/last image into the scene by N frames). Can set to 0, but will result in some video files failing to extract the very last frame.

We should remove the last sentence clause, and instead just do our best to get all of the requested frames. If we can't, instead of emitting no files, we should just emit the frame closest to it (or last).