add avcodec_get_subtitle_defaults() to initialize AVSubtitle struct

add avcodec_get_subtitle_defaults() to initialize AVSubtitle struct

Call this new function before decode() to replace the custom and
inconsistant initialization in various decoders.
This function is equivalent to avcodec_get_frame_defaults() for AVFrame.

Signed-off-by: Aurelien Jacobs <aurel@gnuage.org>