Implement sawtooth wave

src/sound.cpp

@@ -78,7 +78,7 @@ static inline float triangle_wave(float idx) {
 }
 
 static inline float sawtooth_wave(float idx) {
-  return 0;
+  return std::abs(fmod(idx * 2, 1.0f)) * 2 - 1;
 }
 
 void WaveGeneratorSource::gen_sound_data(bool play_sound, short buffer[], size_t buffer_size) {