av_log callback handling overhaul

Move av_log callback handling from vd_ffmpeg.c to a new file
av_log.c and install the callback immediately when starting the
program. Main functionality improvements of the new code:

 - The old version only installed the callback when opening an FFmpeg
   video decoder. If nothing had triggered that then av_log() messages
   from other sources (libavformat, audio decoding, swscale usage)
   bypassed MPlayer's output system completely. Now the callback is
   always installed.

 - Current av_log message severity levels are handled correctly. The
   old code used MSGL_ERR for some messages that should be MSGL_V.

 - Message type is now set for libavformat contexts
   (MSGT_DEMUXER / MSGT_MUXER).

 - The old code did "mp_msg_test(type, mp_level)" before actually
   determining the type, so that it always used MSGT_FIXME. This led
   to some messages being incorrectly dropped in case the user
   had specified module-specific verbosity levels. The old check in
   question was originally motivated by performance problems when
   there were a lot of callbacks; however it's not clear whether the
   part about it skipping the type determination was intentional (most
   of the performance problems must have come from the way the
   original code used snprintf) and in my tests current FFmpeg
   libraries have not generated unreasonable amounts of callbacks
   anyway.

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