Create a good default name for saving individual frames of a movie file loaded
as an image datablock, instead of the movie file name.
Changes ImBuf to store the frame separate from the filepath, to implement this.
Seems more clear for ImBuf.filepath to be an actual filepath anyway.
Thanks to Jesse and Aras for investigating this bug.
Pull Request: https://projects.blender.org/blender/blender/pulls/140471
5b1126da66