jpeg: Restore grayscale image support
jpeg: Restore grayscale image support

Commit c2a40a92fe3df4111ed9da51fe3368c079b86926 restricts the jpeg
loader to files with 3 or 4 color components.

Also allow JCS_GRAYSCALE images with only 1 color component.
These grayscale images are supported through explode_gray_into_buf.

https://bugzilla.gnome.org/show_bug.cgi?id=785171

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