trivial: Allow zero width and height for as_screenshot_get_image()
trivial: Allow zero width and height for as_screenshot_get_image()

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