trivial: Never report padding on images with no alpha channel
trivial: Never report padding on images with no alpha channel

ikeydoherty/appstream-glib,hughsie/appstream-glib,kalev/appstream-glib,mitya57/appstream-glib,kalev/appstream-glib,ximion/appstream-glib,superm1/appstream-glib,hughsie/appstream-glib,ximion/appstream-glib,mitya57/appstream-glib,ikeydoherty/appstream-glib,hughsie/appstream-glib,ximion/appstream-glib,superm1/appstream-glib