[xcode] Go back to wav as default format for RSP/DAAP transcoding

But add "prefer_format" config option for mp3, incl. a check for an encoder.
The config option is currently not added to owntone.conf.in, so is undocumented
until decided what to do about possible alac support.

src/conffile.c

@@ -111,6 +111,7 @@ static cfg_opt_t sec_library[] =
     CFG_BOOL("itunes_smartpl", cfg_false, CFGF_NONE),
     CFG_STR_LIST("no_decode", NULL, CFGF_NONE),
     CFG_STR_LIST("force_decode", NULL, CFGF_NONE),
+    CFG_STR("prefer_format", NULL, CFGF_NONE),
     CFG_BOOL("pipe_autostart", cfg_true, CFGF_NONE),
     CFG_INT("pipe_sample_rate", 44100, CFGF_NONE),
     CFG_INT("pipe_bits_per_sample", 16, CFGF_NONE),

src/transcode.c

@@ -1804,9 +1804,12 @@ transcode_decode_setup_raw(enum transcode_profile profile, struct media_quality
 enum transcode_profile
 transcode_needed(const char *user_agent, const char *client_codecs, char *file_codectype)
 {
-  char *codectype;
+  const char *codectype;
+  const char *prefer_format;
   cfg_t *lib;
   bool force_xcode;
+  bool supports_mpeg;
+  bool supports_wav;
   int count;
   int i;
 
@@ -1862,10 +1865,25 @@ transcode_needed(const char *user_agent, const char *client_codecs, char *file_c
 
   if (!force_xcode && strstr(client_codecs, file_codectype))
     return XCODE_NONE;
-  else if (strstr(client_codecs, "mpeg"))
-    return XCODE_MP3;
-  else if (strstr(client_codecs, "wav"))
+
+  supports_mpeg = strstr(client_codecs, "mpeg") && avcodec_find_encoder(AV_CODEC_ID_MP3);
+  supports_wav = strstr(client_codecs, "wav");
+
+  prefer_format = cfg_getstr(lib, "prefer_format");
+  if (prefer_format)
+    {
+      if (strcmp(prefer_format, "wav") == 0 && supports_wav)
+	return XCODE_WAV;
+      else if (strcmp(prefer_format, "mpeg") == 0 && supports_mpeg)
+	return XCODE_MP3;
+    }
+
+  // This order determines the default if user didn't configure a preference.
+  // The lossless formats are given highest preference.
+  if (supports_wav)
     return XCODE_WAV;
+  else if (supports_mpeg)
+    return XCODE_MP3;
   else
     return XCODE_UNKNOWN;
 }