Cairo uses knockout groups to put its fallback images into PDF files, and cairo would be happier if poppler would just use SOURCE to draw those groups.
This could probably be done if poppler looked at the knockout group and saw that it just consists of drawing one rectangular image, (so it knows the "shape" is a rectangle and doesn't need to compute the shape and draw with DEST_OUT and ADD to get the shape).
Good luck, Jeff!
It looks like this would have to be done with EXTEND_PAD and clipping to a rectangle. The pdf backend doesn't seem to have proper support for EXTEND_PAD and if it was fixed, it seems likely using EXTEND_PAD would cause a fallback. So there really isn't good api for painting the knockout groups with SOURCE.