This script adds the selected layers in reverse order to the image.
The script is started by the Image > Mirror layers menu. This menu is disabled
if there are no selected layers or only one selected layer in the image
(this script expects multiple layers as input).
A,B,C,D,E (from the bottom):
A,B,C,D,E,D,C,B.
This is what is needed for a back-and-forth animation,
where the frames at both ends of the movement are only played once.A,B,C,D,E,E,D,C,B,A.A,B,C,D,E,D,C,B,A