VGM export: write resets and GD3 tag

may be non-standard compliant (yet)
also it crashes foobar2000 for some reason but this will be fixed

src/engine/platform/saa.cpp

@@ -262,6 +262,9 @@ void DivPlatformSAA1099::reset() {
     chan[i]=DivPlatformSAA1099::Channel();
     chan[i].vol=0x0f;
   }
+  if (dumpWrites) {
+    addWrite(0xffffffff,0);
+  }
 
   lastBusy=60;
   dacMode=0;