[9] Review request for 8135304: ERROR_MEDIA_INVALID when playing MP4 file

[Alexander Matveev]

Hi Kirill,

https://bugs.openjdk.java.net/browse/JDK-8135304
http://cr.openjdk.java.net/~almatvee/8135304/webrev.00/

This issue affects all MP4 files with header at the end of file and when 
file is larger then 10 megs.

- javasource was not supporting GST_QUERY_SEEKING and thus qtdemux 
assumed that file is not seekable and thus was not able to find header.
- After fixing above file still was not played, because javasource was 
not able to send NEW SEGMENT after it send EOS. Fixed by flushing source 
pad on javasource.
- Also, we need to clear pending events in progressbuffer when seek is 
performed, otherwise it breaks playback intermittently by sending wrong 
sequence of events.

Thanks,
Alexander