libavformat: output cues for each subtitle block in MKV muxer

Signed-off-by: John Peebles <johnpeeb@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Use rc_max_rate in av_find_best_stream() if bitrate == 0.

Fixes ticket #2042.
lavc/utvideoenc: Enable frame-threading.
lavc/tiffenc: Enable frame-threading.
lavc/ljpegenc: Enable frame-threading.
lavc/lclenc: Enable frame-threading.
lavc/jpeglsenc: Enable frame-threading.