libx264: fix overwriting presets with flags2

This fixes overwriting x264 preset settings for flags2 based options.
See https://ffmpeg.org/trac/ffmpeg/ticket/448

libavcodec/libx264.c

@@ -358,6 +358,7 @@ static av_cold int X264_init(AVCodecContext *avctx)
         if (avctx->partitions & X264_PART_B8X8)
             x4->params.analyse.inter |= X264_ANALYSE_BSUB16x16;
     }
+    if (avctx->flags2) {
         x4->params.analyse.b_ssim = avctx->flags2 & CODEC_FLAG2_SSIM;
         x4->params.b_intra_refresh = avctx->flags2 & CODEC_FLAG2_INTRA_REFRESH;
         x4->params.i_bframe_pyramid = avctx->flags2 & CODEC_FLAG2_BPYRAMID ? X264_B_PYRAMID_NORMAL : X264_B_PYRAMID_NONE;
@@ -368,6 +369,7 @@ static av_cold int X264_init(AVCodecContext *avctx)
         x4->params.b_aud                      = avctx->flags2 & CODEC_FLAG2_AUD;
         x4->params.analyse.b_psy              = avctx->flags2 & CODEC_FLAG2_PSY;
         x4->params.rc.b_mb_tree               = !!(avctx->flags2 & CODEC_FLAG2_MBTREE);
+    }
 #endif
 
     if (avctx->me_method == ME_EPZS)
@@ -578,6 +580,7 @@ static const AVClass class = {
 static const AVCodecDefault x264_defaults[] = {
     { "b",                "0" },
     { "bf",               "-1" },
+    { "flags2",           "0" },
     { "g",                "-1" },
     { "qmin",             "-1" },
     { "qmax",             "-1" },