Soak a cloth in acetone.
Lay the cloth over the melted polyester. Tape it directly to the glass with packing or masking tape. Let it sit there for several hours.
Spray the cloth with acetone as it dries out.
Remove the cloth and use it to scrub the window to remove the rest of the softened melted polyester as well as any tape residue that is was left behind.
If the polyester was not completely removed with the acetone, wet the surface with water and scrape the melted polyester off the glass with a very sharp razor blade. Work until the melted polyester is gone. Don’t use dull blades as these have a higher tendency to scratch glass windows.