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
libav 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.

This contains further changes and simplifications by Michael Niedermayer
and Martin Storsjö.
Signed-off-by: Martin Storsjö <martin@martin.st>