* block: added

 * block: added
    - BLOCK_FLAG_CORRUPTED : signal corrupted data (do not use anymore
 BLOCK_FLAG_DISCONTINUITY in that case)
    - BLOCK_FLAG_PREROLL : mark this block to be decoded (no matter what).
 * ffmpeg, libmpeg2: support BLOCK_FLAG_PREROLL (ie disable frame dropping).
 * input: added ES_OUT_SET_NEXT_DISPLAY_TIME to ease the work for preroll
 (untested).
 * mp4: added support for CTTS table (pts, needed for h264+bframe).
 * decoders: for now handle discontinuity and corrupted block the same way.