avformat/utils: Move demuxing code out into a new file

libavformat/utils.c has over 4800 lines and is supposed to contain
"various utility functions for use within FFmpeg". In reality it
contains all that and the whole demuxing core of libavformat.
This is especially bad, because said file includes the FFMPEG_VERSION
(the git commit sha) so that it is rebuilt whenever the commit HEAD
points to changes. Therefore this commit makes it smaller by moving
the demuxing code out to a new file, demux.c (in analogy to mux.c
for the muxing code).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/Makefile

@@ -8,6 +8,7 @@ HEADERS = avformat.h                                                    \
 OBJS = allformats.o         \
        avio.o               \
        aviobuf.o            \
+       demux.o              \
        dump.o               \
        format.o             \
        id3v1.o              \

libavformat/internal.h

@@ -652,6 +652,8 @@ void ff_reduce_index(AVFormatContext *s, int stream_index);
 
 enum AVCodecID ff_guess_image2_codec(const char *filename);
 
+const AVCodec *ff_find_decoder(AVFormatContext *s, const AVStream *st,
+                               enum AVCodecID codec_id);
 /**
  * Perform a binary search using av_index_search_timestamp() and
  * AVInputFormat.read_timestamp().