Add VRC6 support

2 Pulse channels:
- 8 level pulse duty cycle, DAC mode(just ignores duty cycle)/pulse wave mode, 4 bit volume.
- Furnace support PCM playback in pulse channels with duty cycle ignore mode.

Sawtooth:
- nothing but 6 bit volume (8 bit accumulator in technically) and 12 bit frequency (periodic).

VRC6 instrument:
- 6 bit Volume macro for finer sawtooth volume handling, also 3 bit Duty cycle macro for pulse channels.

Duty, PCM mode command and Duty macro affects for pulse channel only.

src/engine/dispatchContainer.cpp

@@ -48,6 +48,7 @@
 #include "platform/bubsyswsg.h"
 #include "platform/pet.h"
 #include "platform/vic20.h"
+#include "platform/vrc6.h"
 #include "platform/dummy.h"
 #include "../ta-log.h"
 #include "song.h"
@@ -283,6 +284,9 @@ void DivDispatchContainer::init(DivSystem sys, DivEngine* eng, int chanCount, do
     case DIV_SYSTEM_VIC20:
       dispatch=new DivPlatformVIC20;
       break;
+    case DIV_SYSTEM_VRC6:
+      dispatch=new DivPlatformVRC6;
+      break;
     default:
       logW("this system is not supported yet! using dummy platform.\n");
       dispatch=new DivPlatformDummy;