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/playback.cpp

@@ -330,6 +330,18 @@ bool DivEngine::perSystemEffect(int ch, unsigned char effect, unsigned char effe
           return false;
       }
       break;
+    case DIV_SYSTEM_VRC6:
+      switch (effect) {
+        case 0x12: // duty or noise mode
+          dispatchCmd(DivCommand(DIV_CMD_STD_NOISE_MODE,ch,effectVal));
+          break;
+        case 0x17: // PCM enable
+          dispatchCmd(DivCommand(DIV_CMD_SAMPLE_MODE,ch,(effectVal>0)));
+          break;
+        default:
+          return false;
+      }
+      break;
     default:
       return false;
   }