Add Gaussian (SNES) audio interpolation

Probably not a good choice for most DS games unless you really want a
very soft sound, but it could be fun if you wanted to run lolSnes in
melonDS :p

src/SPU.h

@@ -40,6 +40,7 @@ enum class AudioInterpolation
     Linear,
     Cosine,
     Cubic,
+    SNESGaussian
 };
 
 class SPUChannel