vd_lavc: set AVCodecContext.time_base to forced time base

This is a bit sketchy, as there isn't a truly standard way to
communicate the timebase.

video/decode/vd_lavc.c

@@ -445,6 +445,9 @@ static void init_avctx(struct dec_video *vd, const char *decoder,
     avctx->codec_type = AVMEDIA_TYPE_VIDEO;
     avctx->codec_id = lavc_codec->id;
 
+    if (ctx->codec_timebase.num)
+        avctx->time_base = ctx->codec_timebase;
+
     avctx->refcounted_frames = 1;
     ctx->pic = av_frame_alloc();
     if (!ctx->pic)