Fix: Honor default soundfont for FluidSynth

Debian now provides a default soundfont for FluidSynth via its alternatives system.

In short, FluidSynth is configured to look for `/usr/share/sounds/sf3/default-GM.sf3` as its default soundfont, and each soundfront package (FluidR3, OPL-3, MuseScore...) may provide or override this symlink. By default, FluidSynth is installed on Debian with the `TimGM6mb` soundfont by default due to its limited size.

See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=929185 for further details.

src/music/fluidsynth.cpp

@@ -29,7 +29,15 @@ static FMusicDriver_FluidSynth iFMusicDriver_FluidSynth;
 
 /** List of sound fonts to try by default. */
 static const char *default_sf[] = {
-	/* Debian/Ubuntu/OpenSUSE preferred */
+	/* FluidSynth preferred */
+	/* See: https://www.fluidsynth.org/api/settings_synth.html#settings_synth_default-soundfont */
+	"/usr/share/soundfonts/default.sf2",
+
+	/* Debian/Ubuntu preferred */
+	/* See: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=929185 */
+	"/usr/share/sounds/sf3/default-GM.sf3",
+
+	/* OpenSUSE preferred */
 	"/usr/share/sounds/sf2/FluidR3_GM.sf2",
 
 	/* RedHat/Fedora/Arch preferred */