Latest ffmpeg has removed avcodec_get_context_defaults(), and its
documentation says a new AVCodecContext should be allocated for this
purpose.  The pointer returned by avcodec_find_decoder() is now
const-qualified so we also need to adjust for it.  And, AVCOL_RANGE_MPEG
is now rejected with strict_std_compliance > FF_COMPLIANCE_UNOFFICIAL.

Part-of: <!1531>