Add OpenGL backend: OpenGLDevice class

- Created OpenGLDevice class for window and OpenGL context management
- Added CreateWindow() and InitializeWithExistingWindow() methods
- Integrated with GLFW and GLAD
- Added to tests/OpenGL as test target

This is the first step in building the OpenGL RHI backend parallel to D3D12.

tests/OpenGL/OpenGLDevice.h

@@ -0,0 +1,42 @@
+#pragma once
+
+#include <string>
+#include <GLFW/glfw3.h>
+
+namespace XCEngine {
+namespace RHI {
+
+struct OpenGLDeviceInfo {
+    std::string vendor;
+    std::string renderer;
+    std::string version;
+    int majorVersion;
+    int minorVersion;
+};
+
+class OpenGLDevice {
+public:
+    OpenGLDevice();
+    ~OpenGLDevice();
+
+    bool CreateWindow(int width, int height, const char* title, bool enableDebug = false);
+    bool InitializeWithExistingWindow(GLFWwindow* window);
+    void Shutdown();
+
+    GLFWwindow* GetWindow() const { return m_window; }
+    const OpenGLDeviceInfo& GetDeviceInfo() const { return m_deviceInfo; }
+
+    void SwapBuffers();
+    bool PollEvents();
+    void SetShouldClose(bool shouldClose);
+    bool ShouldClose() const;
+
+private:
+    GLFWwindow* m_window;
+    OpenGLDeviceInfo m_deviceInfo;
+    bool m_initialized;
+    bool m_ownsWindow;
+};
+
+} // namespace RHI
+} // namespace XCEngine