VGM export: add speed drift compensation setting

it appears this is the only way to get exports to play at the correct
speed in DeadFish's VGM player for Genesis

src/gui/exportOptions.cpp

@@ -239,6 +239,16 @@ void FurnaceGUI::drawExportVGM(bool onWindow) {
     }
   }
 
+  ImGui::Text(_("speed drift compensation:"));
+  if (ImGui::RadioButton(_("none"),vgmExportCorrectedRate==44100)) {
+    vgmExportCorrectedRate=44100;
+  }
+  // as tested on a Model 1 Genesis (VA6, USA):
+  // 0.97841613336995507871 slower, 1.02206000687632131440 longer
+  if (ImGui::RadioButton(_("DeadFish VgmPlay (1.02×)"),vgmExportCorrectedRate==43148)) {
+    vgmExportCorrectedRate=43148;
+  }
+
   if (hasOneAtLeast) {
     if (onWindow) {
       ImGui::Separator();

src/gui/gui.cpp

@@ -5654,7 +5654,7 @@ bool FurnaceGUI::loop() {
               break;
             }
             case GUI_FILE_EXPORT_VGM: {
-              SafeWriter* w=e->saveVGM(willExport,vgmExportLoop,vgmExportVersion,vgmExportPatternHints,vgmExportDirectStream,vgmExportTrailingTicks,vgmExportDPCM07);
+              SafeWriter* w=e->saveVGM(willExport,vgmExportLoop,vgmExportVersion,vgmExportPatternHints,vgmExportDirectStream,vgmExportTrailingTicks,vgmExportDPCM07,vgmExportCorrectedRate);
               if (w!=NULL) {
                 FILE* f=ps_fopen(copyOfName.c_str(),"wb");
                 if (f!=NULL) {
@@ -8478,6 +8478,7 @@ FurnaceGUI::FurnaceGUI():
   debugFFT(false),
   vgmExportVersion(0x171),
   vgmExportTrailingTicks(-1),
+  vgmExportCorrectedRate(44100),
   drawHalt(10),
   macroPointSize(16),
   waveEditStyle(0),

src/gui/gui.h

@@ -1687,6 +1687,7 @@ class FurnaceGUI {
   bool willExport[DIV_MAX_CHIPS];
   int vgmExportVersion;
   int vgmExportTrailingTicks;
+  int vgmExportCorrectedRate;
   int cvHiScore;
   int drawHalt;
   int macroPointSize;