Cleaning paint/glue stuck on glass
Many (20? 25?) years ago, my sister and I had a fun craft project where we made window decals out of some kind of glitter puff paint or maybe glitter mixed with Elmer's glue, and stuck them to my mom's living room window. See photo for the artistry. It's lived there since, on a window that gets several hours of direct sunlight every single day in the heat of central California.
We've tried scraping it off with a razor without much luck. You'd think it would be brittle and flake off but no, it's basically melted and become one with the glass and is rock hard but somehow also rubbery and not something that can be cut into.
My mom is trying to sell her house and if necessary we'll replace the whole window but would like to avoid that.
Please help. Any suggestions? Should we try something like goo gone? Paint stripper? Something scratchy that won't mess up the glass? I have no idea where to begin.