obs-ffmpeg: Use core count for output threads by default

Recent FFmpeg versions seem to have changed the default thread count for
most codecs to 1.  Use the max core count by default as it was
originally.

plugins/obs-ffmpeg/obs-ffmpeg-output.c

@@ -242,7 +242,12 @@ static bool create_video_stream(struct ffmpeg_data *data)
 	context->pix_fmt        = closest_format;
 	context->colorspace     = data->config.color_space;
 	context->color_range    = data->config.color_range;
-	context->thread_count   = 0;
+
+	if (context->thread_count == 1) {
+		int caps = context->codec->capabilities;
+		context->thread_count = !!(caps & AV_CODEC_CAP_AUTO_THREADS) ?
+			0 : av_cpu_count();
+	}
 
 	data->video->time_base = context->time_base;