splitmuxsink: Added caption_%u pad template
splitmuxsink: Added caption_%u pad template

For closed-caption-enabled muxers (e.g. qtmux)

GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good