Prepare to add hasSamplePtrHeader and hasSampleInstHeader in dispatch (WIP)

for refresh sample memory when loop/end pointer and instrument parameter changed. Also, this PR has minor code style fixes and add warning in MultiPCM sample map usage.

src/engine/platform/opl.cpp

@@ -3244,6 +3244,14 @@ size_t DivPlatformOPL::getSampleMemUsage(int index) {
           (index==0 && adpcmChan>=0)?adpcmBMemLen:0;
 }
 
+bool DivPlatformOPL::hasSamplePtrHeader(int index) {
+  return (index==0 && pcmChanOffs>=0);
+}
+
+bool DivPlatformOPL::hasSampleInstHeader(int index) {
+  return (index==0 && pcmChanOffs>=0);
+}
+
 bool DivPlatformOPL::isSampleLoaded(int index, int sample) {
   if (index!=0) return false;
   if (sample<0 || sample>32767) return false;