console: Load companion .m3u/.m3u8 for subsong metadata

The bundled GME library includes a full M3u_Playlist parser that
understands the NEZplug extended m3u format (with ::TYPE and $track
syntax), but the code to auto-load a companion .m3u was disabled.

Re-enable it using the Audacious StringBuf API. When a game music
file is loaded, look for a matching .m3u8 (preferred) or .m3u file
in the same directory and feed it to the emulator. This provides
track names, durations, and correct subsong mapping for KSS, NSF,
GBS, HES, and other multi-track formats.

Author: ramonsmits

src/console/Audacious_Driver.cc

@@ -120,24 +120,22 @@ int ConsoleFileHandler::load(int sample_rate)
 
     log_warning(m_emu);
 
-#if 0
-    // load .m3u from same directory( replace/add extension with ".m3u")
-    char *m3u_path = g_strdup(m_path);
-    char *ext = strrchr(m3u_path, '.');
-    if (ext == nullptr)
-    {
-        ext = g_strdup_printf("%s.m3u", m3u_path);
-        g_free(m3u_path);
-        m3u_path = ext;
-    }
+    // load companion .m3u or .m3u8 from same directory
+    StringBuf base_path = str_copy(m_path);
+    const char *dot = strrchr(base_path, '.');
+    if (dot)
+        base_path.resize(dot - base_path);
 
     Vfs_File_Reader m3u;
+    StringBuf m3u_path = str_concat({base_path, ".m3u8"});
+    if (m3u.open(m3u_path))
+        m3u_path = str_concat({base_path, ".m3u"});
+
     if (!m3u.open(m3u_path))
     {
-        if (log_err(m_emu->load_m3u(m3u))) // TODO: fail if m3u can't be loaded?
-            log_warning(m_emu); // this will log line number of first problem in m3u
+        if (log_err(m_emu->load_m3u(m3u)))
+            log_warning(m_emu);
     }
-#endif
 
     return 0;
 }