Resolves #7879

From my reading, Page 10 of the MPO specification shows that the APP1 marker should contain EXIF data. However, a user has found an image where it does not. This lead to an error when the not-EXIF data was treated as EXIF data, and an error when the size of the second frame wasn't read from the EXIF data.

This PR updates the code to use JpegImagePlugin's reading of markers. This allows for the possibility of APP1 markers without EXIF data, and reads the size from SOF-like markers instead of from EXIF data.