Add OpenGL backend core classes: Buffer, VertexArray

- Added OpenGLBuffer class for VBO/IBO management
- Added OpenGLVertexArray class for VAO management
- Updated CMakeLists.txt to include new source files

tests/OpenGL/OpenGLVertexArray.h

@@ -0,0 +1,44 @@
+#pragma once
+
+#include <string>
+#include <GLFW/glfw3.h>
+#include <vector>
+
+namespace XCEngine {
+namespace RHI {
+
+struct VertexAttribute {
+    unsigned int index;
+    int count;
+    unsigned int type;
+    bool normalized;
+    size_t stride;
+    size_t offset;
+};
+
+class OpenGLVertexArray {
+public:
+    OpenGLVertexArray();
+    ~OpenGLVertexArray();
+
+    bool Initialize();
+    void AddVertexBuffer(unsigned int buffer, const VertexAttribute& attribute);
+    void SetIndexBuffer(unsigned int buffer, unsigned int type);
+    void Shutdown();
+
+    void Bind() const;
+    void Unbind() const;
+
+    unsigned int GetID() const { return m_vao; }
+    unsigned int GetIndexBuffer() const { return m_indexBuffer; }
+    unsigned int GetIndexCount() const { return m_indexCount; }
+
+private:
+    unsigned int m_vao;
+    unsigned int m_indexBuffer;
+    unsigned int m_indexCount;
+    int m_vertexBufferCount;
+};
+
+} // namespace RHI
+} // namespace XCEngine