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/gui/presets.cpp

@@ -191,6 +191,13 @@ void FurnaceGUI::initSystemPresets() {
       0
     }
   ));
+  cat.systems.push_back(FurnaceGUISysDef(
+    "NES with Konami VRC6", {
+      DIV_SYSTEM_NES, 64, 0, 0,
+      DIV_SYSTEM_VRC6, 64, 0, 0,
+      0
+    }
+  ));
   cat.systems.push_back(FurnaceGUISysDef(
     "NES with Konami VRC7", {
       DIV_SYSTEM_NES, 64, 0, 0,