queue2: Handle buffering on sparse streams
Submitted by Edward Hervey
If an adaptive demuxer (or any other source) provides a sparse stream (subtitles or captions), queue2 goes nuts constantly going posting buffering messages from 0% to 100% and back.
If we have a SPARSE stream, we should just post 100% buffering.
And this assumes that upstream elements properly set the stream flags of course :)