File libopenh264enc.c has been modified so that the encoder uses av_log()
to log messages (error, warning, info, etc.) instead of logging them
directly to stderr.  At the time the encoder is created, the current
ffmpeg log level is mapped to an equivalent libopenh264 log level.  This
log level, and a message logging function that invokes av_log() to
actually log messages, are then set on the encoder.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>