Remove audio resampling from avformat producer.
Drop support for FFmpeg v0.5 and v0.6 and require swscale.

This is cleanup work to make way for more version handling to handle
current git master of libav that removes deprecated APIs.
Add support for libavformat and libavcodec major version 55.
Restore avresample filter when build against FFmpeg.

This provides a LGPL audio resampler, but it is using deprecated APIs.
Support for swresample or libav avresampler is another project.
Improve frame rate detection with libav master.