gst-libav: fix build with ffmpeg-5.0.0
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.