Draft: downloadbuffer: Report 100% buffering after seek if EOS has arrived

A fix for https://bugs.webkit.org/show_bug.cgi?id=196198 needs this change to work. WebKit GStreamer code pauses the player when the downloadbuffer goes below a given threshold and only unpauses it when it fills up again.

If the server stalls the download (on purpose in this specific test) an eventual EOS is generated by the source element, so the downloadbuffer is sure that no more data will ever come. If a seek is done after that, the WebKit player code will still be paused and the expected buffering level won't be ever reported.

This patch detects that condition and explicitly reports 100% buffering after seek as long as EOS had been detected in the sink pad in the past.