decoder: implement pf_flush for all decoders/packetizers
For now, a lot of decoder/packetizer are also flushing on BLOCK_FLAG_DISCONTINUITY flag. Some others are also flushing on BLOCK_FLAG_CORRUPTED flag (omxil, videotoolbox, avcodec audio). This patch doesn't change the current behavior. But maybe we shouldn't flush anymore on DISCONTINUOUS/CORRUPTED.