Support newer version of libavformat

In later version, av_find_best_stream fills in a pointer to *const*
AVCodec* and I need a preprocessor check to make sure I can handle both
the new version and the old one void gives me. (To be fair, it should
have been a const all along, so I'm not annoyed at the change.)

src/input.c

@@ -400,7 +400,11 @@ static int open_video( struct input_source* source, char** error ) {
 		return -1;
 	}
 
+#if LIBAVFORMAT_VERSION_MAJOR >= 59
+	const AVCodec* decoder;
+#else
 	AVCodec* decoder;
+#endif
 	int stream_number = av_find_best_stream( format_context,
 		AVMEDIA_TYPE_VIDEO, -1, -1, &decoder, 0 );
 	if ( 0 > stream_number ) {