When the slideshow is deactivated (not shown to the stream), makes it so
that it can either continue playing regardless, pause, or restart the
slideshow when the source is activated (shown to the stream) again.
Split from jp9000/obs-studio#976
Adds an option that allows the user to specify the size (or aspect) of
the bounding source size which all the images are then scaled in to.
Acts similarly to "bounding box" of scene items, but for the slide show
and its sub-sources.