Refactor mapping to the native structs

imgui-binding/src/main/native/jni_binding_struct.cpp

@@ -0,0 +1,17 @@
+#include "jni_binding_struct.h"
+
+jfieldID imGuiStructPtrID;
+
+namespace Jni
+{
+    void InitBindingStruct(JNIEnv* env)
+    {
+        jclass jImGuiStructClass = env->FindClass("imgui/binding/ImGuiStruct");
+        imGuiStructPtrID = env->GetFieldID(jImGuiStructClass, "ptr", "J");
+    }
+
+    jfieldID GetBindingStructPtrID()
+    {
+        return imGuiStructPtrID;
+    }
+}

imgui-binding/src/main/native/jni_binding_struct.h

@@ -0,0 +1,17 @@
+#include <jni.h>
+
+#ifndef JNI_BINDING_STRUCT_H
+#define JNI_BINDING_STRUCT_H
+
+#ifndef STRUCT_PTR
+#define STRUCT_PTR env->GetLongField(object, Jni::GetBindingStructPtrID())
+#endif
+
+namespace Jni
+{
+    void InitBindingStruct(JNIEnv* env);
+
+    jfieldID GetBindingStructPtrID();
+}
+
+#endif