Allow user and application volume adjustments to both apply

Comment from ripsaw8080 (2019-11-10 @ 08:03):

It's important to consider features/functionality when looking for ways
to reduce processing. The MSCDEX-based volume control was made separate
of the DOSBox mixer for two reasons: 1) the SB mixer interface CD volume
control is tied to the DOSBox mixer by default (can be disabled with
sbmixer=false) and the two methods would be in conflict; 2) the user can
adjust the mixer volume on top of what the game does with the MSCDEX
volume, in other words a relative adjustment rather than an override.

include/mixer.h

@@ -48,6 +48,7 @@ class MixerChannel {
 public:
 	void SetVolume(float _left,float _right);
 	void SetScale( float f );
+	void SetScale( float _left, float _right_);
 	void UpdateVolume(void);
 	void SetFreq(Bitu _freq);
 	void Mix(Bitu _needed);
@@ -79,7 +80,7 @@ class MixerChannel {
 	void Enable(bool _yesno);
 	MIXER_Handler handler;
 	float volmain[2];
-	float scale;
+	float scale[2];
 	Bit32s volmul[2];
 
 	//This gets added the frequency counter each mixer step

src/dos/cdrom_image.cpp

@@ -538,7 +538,7 @@ void CDROM_Interface_Image::ChannelControl(TCtrl ctrl)
 	if (player.ctrlUsed && player.channel != NULL) {
 		const float leftVolumePercent = static_cast<float>(player.ctrlData.vol[0] / 255.0);
 		const float rightVolumePercent = static_cast<float>(player.ctrlData.vol[1] / 255.0);
-		player.channel->SetVolume(leftVolumePercent, rightVolumePercent);
+		player.channel->SetScale(leftVolumePercent, rightVolumePercent);
 #ifdef DEBUG
 		LOG_MSG("%s CDROM: Game set the left and right volume to %3.0f%% and %3.0f%%, respectively",
 		        get_time(), leftVolumePercent * 100, rightVolumePercent * 100);

src/hardware/mixer.cpp

@@ -92,11 +92,11 @@ Bit8u MixTemp[MIXER_BUFSIZE];
 
 MixerChannel * MIXER_AddChannel(MIXER_Handler handler,Bitu freq,const char * name) {
 	MixerChannel * chan=new MixerChannel();
-	chan->scale = 1.0;
 	chan->handler=handler;
 	chan->name=name;
 	chan->SetFreq(freq);
 	chan->next=mixer.channels;
+	chan->SetScale(1.0);
 	chan->SetVolume(1,1);
 	chan->enabled=false;
 	chan->interpolate = false;
@@ -128,8 +128,8 @@ void MIXER_DelChannel(MixerChannel* delchan) {
 }
 
 void MixerChannel::UpdateVolume(void) {
-	volmul[0]=(Bits)((1 << MIXER_VOLSHIFT)*scale*volmain[0]*mixer.mastervol[0]);
-	volmul[1]=(Bits)((1 << MIXER_VOLSHIFT)*scale*volmain[1]*mixer.mastervol[1]);
+	volmul[0]=(Bits)((1 << MIXER_VOLSHIFT)*scale[0]*volmain[0]*mixer.mastervol[0]);
+	volmul[1]=(Bits)((1 << MIXER_VOLSHIFT)*scale[1]*volmain[1]*mixer.mastervol[1]);
 }
 
 void MixerChannel::SetVolume(float _left,float _right) {
@@ -139,7 +139,14 @@ void MixerChannel::SetVolume(float _left,float _right) {
 }
 
 void MixerChannel::SetScale( float f ) {
-	scale = f;
+	scale[0] = f;
+	scale[1] = f;
+	UpdateVolume();
+}
+
+void MixerChannel::SetScale( float _left, float _right) {
+	scale[0] = _left;
+	scale[1] = _right;
 	UpdateVolume();
 }