Issue #1842: Load Clipping Path metadata from JPEGs

This patch attempts to resolve half of issue 1842. It allows the JPEG plug-in to recognize APP13 markers, which Photoshop uses to store clipping path information in PSD format. The path is imported and converted to a GimpVectors object and attached to the image as a path. The import code is adapted from load_resource_2000 () in /plug-ins/file-psd/psd-image-res-load.c.

The code works with the sample image provided in the enhancement request: https://gitlab.gnome.org/GNOME/gimp/uploads/e1dbddc7fa35f9d3219013a36fb63f40/blue_tux-2.jpg

If anyone has additional sample files, I'd be happy to test those as well. (@Tobias, since you suggested I look into this problem, do you have any other JPEGs with clipping paths?)