Implement master volume & volume interpolation

Does not fix clicking :(

Fixes https://github.com/TakaRikka/dusk/issues/132
Fixes https://github.com/TakaRikka/dusk/issues/128

src/dusk/audio/DuskAudioSystem.cpp

@@ -15,6 +15,7 @@
 
 #include "DuskDsp.hpp"
 #include "JSystem/JAudio2/JASAudioThread.h"
+#include "JSystem/JAudio2/JASDriverIF.h"
 
 // #define DUSK_DUMP_AUDIO
 
@@ -73,6 +74,12 @@ void dusk::audio::Initialize() {
     SDL_ResumeAudioStreamDevice(PlaybackStream);
 }
 
+void dusk::audio::SetMasterVolume(const f32 value) {
+    JASCriticalSection section;
+
+    MasterVolume = value;
+}
+
 void SDLCALL GetNewAudio(
     void*,
     SDL_AudioStream*,
@@ -132,3 +139,11 @@ void RenderAudioSubframe() {
 
     SDL_PutAudioStreamData(PlaybackStream, &OutInterleaveBuffer, sizeof(OutInterleaveBuffer));
 }
+
+u32 dusk::audio::GetResetCount(int channelIdx) {
+    return ChannelAux[channelIdx].resetCount;
+}
+
+f32 dusk::audio::VolumeFromU16(u16 value) {
+    return static_cast<f32>(value) / static_cast<f32>(JASDriver::getChannelLevel_dsp());
+}