REAPER Demo: Check opcode index before running embedded UI commands

The comment at the top of reaper_plugin_fx_embed.h specifies that the
'index' parameter for the embedded UI feature will always be set to
effEditDraw.

examples/Plugins/ReaperEmbeddedViewPluginDemo.h

@@ -67,6 +67,7 @@ JUCE_BEGIN_IGNORE_WARNINGS_GCC_LIKE ("-Wshadow-field-in-constructor",
 #include <pluginterfaces/base/ftypes.h>
 #include <pluginterfaces/base/funknown.h>
 #include <pluginterfaces/vst/ivsthostapplication.h>
+#include <pluginterfaces/vst2.x/aeffect.h>
 
 JUCE_END_IGNORE_WARNINGS_GCC_LIKE
 
@@ -274,11 +275,16 @@ public:
         return 0;
     }
 
-    pointer_sized_int handleVstManufacturerSpecific (int32,
+    pointer_sized_int handleVstManufacturerSpecific (int32 index,
                                                      pointer_sized_int value,
                                                      void* ptr,
                                                      float opt) override
     {
+        // The docstring at the top of reaper_plugin_fx_embed.h specifies
+        // that the index will always be effEditDraw, which is now deprecated.
+        if (index != __effEditDrawDeprecated)
+            return 0;
+
         return (pointer_sized_int) handledEmbeddedUIMessage ((int) opt,
                                                              (Steinberg::TPtrInt) value,
                                                              (Steinberg::TPtrInt) ptr);