obs-ffmpeg: Set AVCodecContext thread_count to 0

For some reason in the FFmpeg output, this AVCodecContext variable is
being set to 1 by FFmpeg itself somewhere, and it's causing a massive
slowdown when encoding with FFmpeg directly.  This should be set to 0 to
specify to use as many threads as necessary.

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

@@ -237,6 +237,7 @@ 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;
 
 	data->video->time_base = context->time_base;