frame-writer: Use superfast instead of ultrafast for libx264/5 (#280)

This aims to default videos to use High profile by default instead of
Constrained Baseline. This reportedly has more web compatibilty and
makes video files slightly smaller.

src/frame-writer.cpp

@@ -50,7 +50,7 @@ void FrameWriter::load_codec_options(AVDictionary **dict)
 
     static const CodecOptions default_x264_options = {
         {"tune", "zerolatency"},
-        {"preset", "ultrafast"},
+        {"preset", "superfast"},
         {"crf", "20"},
     };