Adapt codec PrivateData following libavformat behavior change for AV1

FFmpeg pre 5.0 used to skip the first 4 bytes,
hw codecs does not know how to manage without this.

FFmpeg sw codecs (via dav1d) is supposed to handle this,
but it does not either with our libavcodec implementation.

Fix AV1 decoding for ffmpeg 5+

Source/stream_parser_ffmpeg.c

@@ -381,8 +381,11 @@ serprintf("untouched (!?) vrate=%d; vscale=%d\n", video->rate, video->scale);
 
 				if( codecpar->extradata_size ) {
 					if( codecpar->extradata_size <= sizeof( video->extraData ) ) {
-						video->extraDataSize = codecpar->extradata_size;
-						memcpy( video->extraData, codecpar->extradata, video->extraDataSize  );	
+						// libavformat used to skip the first 4 bytes in av1 private data
+						// hw codecs and our implementation using libavcodec still want this
+						int offset = ( video->format == VIDEO_FORMAT_AV1 ) ? 4 : 0 ;
+						video->extraDataSize = codecpar->extradata_size - offset ;
+						memcpy( video->extraData, codecpar->extradata + offset , video->extraDataSize );
 						if( video->format == VIDEO_FORMAT_H264 && video->extraData[0] == 0x00 ) {
 serprintf("FF: parse H264 SPS\n");
 							// for non-AVCC H264, parse the SPS/PPS here