lavf: allow custom IO for all files

Some (de)muxers open additional files beyond the main IO context.
Currently, they call avio_open() directly, which prevents the caller
from using custom IO for such streams.

This commit adds callbacks to AVFormatContext that default to
avio_open2()/avio_close(), but can be overridden by the caller. All
muxers and demuxers using AVIO are switched to using those callbacks
instead of calling avio_open()/avio_close() directly.

(de)muxers that use the URLProtocol layer directly instead of AVIO
remain unconverted for now. This should be fixed in later commits.

doc/APIchanges

@@ -13,6 +13,10 @@ libavutil:     2015-08-28
 
 API changes, most recent first:
 
+2016-xx-xx - xxxxxxx - lavf 57.3.0 - avformat.h
+  Add AVFormatContext.opaque, io_open and io_close, allowing custom IO
+  for muxers and demuxers that open additional files.
+
 2015-xx-xx - xxxxxxx - lavc 57.12.0 - avcodec.h
   Add AVCodecDescriptor.profiles and avcodec_profile_name().