Flagging BLOCK_FLAG_DISCONTINUITY for every PID in the input when the slider is moved on the GUI or a jump in the stream is made. It is also propagated to the mpegvideo packetizer. The packetizer has a new mode to sync on I-frame boundary after such a jump. It avoids sending some broken frames to the client. (Forward port of branches/0.8.1-jpsaman-thedj revision 11067 and 11842.)