encoder/vorbis: default to quality 3

Don't require a quality or bitrate setting.  If nothing is set, don't
fail startup - just go with a good default.  A quality setting of 3 is
what "oggenc" defaults to as well.

src/encoder/plugins/VorbisEncoderPlugin.cxx

@@ -62,7 +62,7 @@ private:
 };
 
 class PreparedVorbisEncoder final : public PreparedEncoder {
-	float quality;
+	float quality = 3;
 	int bitrate;
 
 public:
@@ -97,7 +97,7 @@ PreparedVorbisEncoder::PreparedVorbisEncoder(const ConfigBlock &block)
 
 		value = block.GetBlockValue("bitrate");
 		if (value == nullptr)
-			throw std::runtime_error("neither bitrate nor quality defined");
+			return;
 
 		quality = -2.0;