multifilesrc: set position as offset from start-index
multifilesrc: set position as offset from start-index

query position in GST_FORMAT_BUFFER returns
offset from start-index rather than index.

https://bugzilla.gnome.org/show_bug.cgi?id=752462

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