Update fix for seek

Update fix for seek

1. Use GST_EVENT_FLUSH_START to help decoder escape from waiting for
   an availabe surface; correspondingly, GST_EVENT_FLUSH_STOP disables
   such special exception.
2. Remove such escape by GST_STATE_CHANGE_PAUSED_TO_READY
   during seek, the status/event happens as following order:
   GST_STATE_CHANGE_PLAYING_TO_PAUSED -> GST_EVENT_FLUSH_START ->
   GST_EVENT_FLUSH_STOP -> GST_STATE_CHANGE_PAUSED_TO_PLAYING
3. Remove such escape by GST_EVENT_EOS.
   EOS is the symptom caused by no surface available, not the reason.
   I'm afraid the original code will miss some frames if EOS comes
   earlier than decoder finishes decoding the sent data.