Prepare to support YM2610B and with Extended channel 3 mode

Allow 8 bit volume for YM2610 ADPCM-B
Remove sample mode macro in YM2610, it's always enabled and seperated channels.

TODO: ADPCM-B is still not implemented, FM Channel 2 is silenced in extended channel 3 configuration

src/engine/platform/ym2610bext.h

@@ -0,0 +1,51 @@
+/**
+ * Furnace Tracker - multi-system chiptune tracker
+ * Copyright (C) 2021-2022 tildearrow and contributors
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "../dispatch.h"
+
+#include "ym2610b.h"
+
+class DivPlatformYM2610BExt: public DivPlatformYM2610B {
+  struct OpChannel {
+    DivMacroInt std;
+    unsigned char freqH, freqL;
+    int freq, baseFreq, pitch;
+    unsigned char ins;
+    signed char konCycles;
+    bool active, insChanged, freqChanged, keyOn, keyOff, portaPause;
+    int vol;
+    unsigned char pan;
+    OpChannel(): freqH(0), freqL(0), freq(0), baseFreq(0), pitch(0), ins(-1), active(false), insChanged(true), freqChanged(false), keyOn(false), keyOff(false), portaPause(false), vol(0), pan(3) {}
+  };
+  OpChannel opChan[4];
+  bool isOpMuted[4];
+  friend void putDispatchChan(void*,int,int);
+  public:
+    int dispatch(DivCommand c);
+    void* getChanState(int chan);
+    void reset();
+    void forceIns();
+    void tick();
+    void muteChannel(int ch, bool mute);
+    bool keyOffAffectsArp(int ch);
+    void notifyInsChange(int ins);
+    int init(DivEngine* parent, int channels, int sugRate, unsigned int flags);
+    void quit();
+    ~DivPlatformYM2610BExt();
+};