ximagesrc: add meta transform function
ximagesrc: add meta transform function

ximage metadata can't be transformed or copied, but provide an empty
transformation function instead of NULL to allow unconditional calling
of metas' transform functions.

https://bugzilla.gnome.org/show_bug.cgi?id=751778

froggatt/gst-plugins-good-m,pexip/gst-plugins-good,Kurento/gst-plugins-good,shelsonjava/gst-plugins-good,hizukiayaka/gst-plugins-good,StreamUtils/gst-plugins-good,chamois94/gst-plugins-good,GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,chamois94/gst-plugins-good,Kurento/gst-plugins-good,shelsonjava/gst-plugins-good,GStreamer/gst-plugins-good,Kurento/gst-plugins-good,ikonst/gst-plugins-good,froggatt/gst-plugins-good-m,rawoul/gst-plugins-good,stfl/gst-plugins-good,froggatt/gst-plugins-good-m,chamois94/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,rawoul/gst-plugins-good,ikonst/gst-plugins-good,sebras/gst-plugins-good,sebras/gst-plugins-good,GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,rawoul/gst-plugins-good,chamois94/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,froggatt/gst-plugins-good-m,sebras/gst-plugins-good,sebras/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,shelsonjava/gst-plugins-good,stfl/gst-plugins-good,GStreamer/gst-plugins-good,ikonst/gst-plugins-good,shelsonjava/gst-plugins-good,GStreamer/gst-plugins-good,hizukiayaka/gst-plugins-good,rawoul/gst-plugins-good,pexip/gst-plugins-good,ikonst/gst-plugins-good