af_lavcac3enc/ad_ffmpeg: convert to use libswresample

Convert to use libswresample.
audio: drop a52dec support

Drop a52dec support, just use FFmpeg.
demux_avi: fix a compiler warning

Fix the following compiler warning by moving a variable:

libmpdemux/demux_avi.c: In function 'demux_open_hack_avi':
libmpdemux/demux_avi.c:867:19: warning: unused variable 'opts' [-Wunused-variable]
vd_ffmpeg: disable dr1

Disable dr1 and eliminate using deprecated FFmpeg APIs. This code may
need to be rewritten in the future.