Used the nearest available frame rate in avcodec encoder.

It fixes #1965.

modules/codec/avcodec/encoder.c

@@ -402,6 +402,17 @@ int OpenEncoder( vlc_object_t *p_this )
 
         p_context->time_base.num = p_enc->fmt_in.video.i_frame_rate_base;
         p_context->time_base.den = p_enc->fmt_in.video.i_frame_rate;
+        if( p_codec->supported_framerates )
+        {
+            AVRational target = {
+                .num = p_enc->fmt_in.video.i_frame_rate,
+                .den = p_enc->fmt_in.video.i_frame_rate_base,
+            };
+            int idx = av_find_nearest_q_idx(target, p_codec->supported_framerates);
+
+            p_context->time_base.num = p_codec->supported_framerates[idx].den;
+            p_context->time_base.den = p_codec->supported_framerates[idx].num;
+        }
 
         /* Defaults from ffmpeg.c */
         p_context->qblur = 0.5;