jpeg: Fix loading truncated incremental inputs
jpeg: Fix loading truncated incremental inputs

When loading incremental inputs through a GdkPixbufLoader, we didn't
have any code replicating the stdio_fill_input_buffer() hack to finish
up a file.

Adding those markers makes libjpeg think that the file was at least
closed properly, and will attempt decoding.

https://bugzilla.gnome.org/show_bug.cgi?id=753605

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf