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.

NEWS

@@ -8,6 +8,8 @@ ver 0.20.12 (not yet released)
   - vorbis: fix Tremor support
 * player
   - log message when decoder is too slow
+* encoder
+  - vorbis: default to quality 3
 * output
   - fix hanging playback with soxr resampler
 

doc/user.xml

@@ -3068,8 +3068,8 @@ run</programlisting>
                 </entry>
                 <entry>
                   Sets the quality for VBR.  -1 is the lowest quality,
-                  10 is the highest quality.  Cannot be used with
-                  <varname>bitrate</varname>.
+                  10 is the highest quality.  Defaults to 3.  Cannot
+                  be used with <varname>bitrate</varname>.
                 </entry>
               </row>
               <row>

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;