splitmuxpartreader: ignore sparse streams when calculating the end offset of a part
splitmuxpartreader: ignore sparse streams when calculating the end offset of a part

A sparse stream's ending timestamp can be considerably smaller
than the ending timestamps of the other streams, which can lead
to skipping considerable time from the next part.

https://bugzilla.gnome.org/show_bug.cgi?id=761086

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