Snes9x - Add the separate echo buffer option. (#836)

Snes9x
- Add the separate echo buffer option.

source/preferences.cpp

@@ -500,6 +500,7 @@ DefaultSettings ()
 	Settings.SoundPlaybackRate = 48000;
 	Settings.SoundInputRate = 31920;
 	Settings.DynamicRateControl = true;
+	Settings.SeparateEchoBuffer = false;
 	
 	// Interpolation Method
 	GCSettings.Interpolation = 0;
@@ -524,7 +525,7 @@ DefaultSettings ()
 	Settings.SuperFXSpeedPerLine = 5823405;
 	
 	Settings.SuperFXClockMultiplier = 100;
-	
+	Settings.OverclockMode = 0;
 	Settings.OneClockCycle = 6;
 	Settings.OneSlowClockCycle = 8;
 	Settings.TwoClockCycles = 12;

source/snes9x/apu/SPC_DSP.cpp

@@ -612,7 +612,7 @@ VOICE_CLOCK(V9_V6_V3) { voice_V9(v); voice_V6(v+1); voice_V3(v+2); }
 //// Echo
 
 // Current echo buffer pointer for left/right channel
-#define ECHO_PTR( ch )      (&m.ram [m.t_echo_ptr + ch * 2])
+#define ECHO_PTR( ch )      ((Settings.SeparateEchoBuffer) ? (&m.separate_echo_buffer [m.t_echo_ptr + ch * 2]) : (&m.ram [m.t_echo_ptr + ch * 2]))
 
 // Sample in echo history buffer, where 0 is the oldest
 #define ECHO_FIR( i )       (m.echo_hist_pos [i])
@@ -881,6 +881,8 @@ void SPC_DSP::soft_reset_common()
 	m.echo_offset        = 0;
 	m.phase              = 0;
 	
+	memset(m.separate_echo_buffer, 0, 0x10000);
+
 	init_counter();
 
 	for (int i = 0; i < voice_count; i++)

source/snes9x/apu/SPC_DSP.h

@@ -199,6 +199,8 @@ private:
 		sample_t* out_end;
 		sample_t* out_begin;
 		sample_t extra [extra_size];
+	
+	uint8_t separate_echo_buffer [0x10000];
 	};
 	state_t m;
 	

source/snes9x/snes9x.h

@@ -300,7 +300,9 @@ struct SSettings
 	bool8	OpenGLEnable;
 	float	SuperFXSpeedPerLine;
 
+	bool8   SeparateEchoBuffer;
 	uint32	SuperFXClockMultiplier;
+	int	OverclockMode;
 	int	OneClockCycle;
 	int	OneSlowClockCycle;
 	int	TwoClockCycles;