vaapi_encode: Clean up the GOP structure configuration

Choose what types of reference frames will be used based on what types
are available, and make the intra-only mode explicit (GOP size one,
which must be used for MJPEG).

libavcodec/vaapi_encode_mjpeg.c

@@ -401,6 +401,7 @@ static av_cold int vaapi_encode_mjpeg_init(AVCodecContext *avctx)
 static const AVCodecDefault vaapi_encode_mjpeg_defaults[] = {
     { "global_quality", "80" },
     { "b",              "0"  },
+    { "g",              "1"  },
     { NULL },
 };