Move OpenGL backend classes from tests/OpenGL to engine/

- Relocated OpenGLDevice, OpenGLShader, OpenGLBuffer, OpenGLVertexArray, OpenGLTexture to engine/
- Updated engine/CMakeLists.txt to include OpenGL backend source files
- Updated tests/OpenGL/CMakeLists.txt to use engine backend
- Added OpenGLTexture class implementation

engine/src/RHI/OpenGL/OpenGLVertexArray.cpp

@@ -0,0 +1,60 @@
+#define GLFW_INCLUDE_NONE
+#include "XCEngine/RHI/OpenGL/OpenGLVertexArray.h"
+#include <glad/glad.h>
+#include <GLFW/glfw3.h>
+
+namespace XCEngine {
+namespace RHI {
+
+OpenGLVertexArray::OpenGLVertexArray()
+    : m_vao(0)
+    , m_indexBuffer(0)
+    , m_indexCount(0)
+    , m_vertexBufferCount(0) {
+}
+
+OpenGLVertexArray::~OpenGLVertexArray() {
+    Shutdown();
+}
+
+bool OpenGLVertexArray::Initialize() {
+    glGenVertexArrays(1, &m_vao);
+    return true;
+}
+
+void OpenGLVertexArray::AddVertexBuffer(unsigned int buffer, const VertexAttribute& attribute) {
+    glBindVertexArray(m_vao);
+    glBindBuffer(GL_ARRAY_BUFFER, buffer);
+    glEnableVertexAttribArray(attribute.index);
+    glVertexAttribPointer(attribute.index, attribute.count, attribute.type, 
+        attribute.normalized ? GL_TRUE : GL_FALSE, 
+        attribute.stride, (void*)attribute.offset);
+    glBindBuffer(GL_ARRAY_BUFFER, 0);
+    glBindVertexArray(0);
+    m_vertexBufferCount++;
+}
+
+void OpenGLVertexArray::SetIndexBuffer(unsigned int buffer, unsigned int type) {
+    glBindVertexArray(m_vao);
+    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, buffer);
+    glBindVertexArray(0);
+    m_indexBuffer = buffer;
+}
+
+void OpenGLVertexArray::Shutdown() {
+    if (m_vao) {
+        glDeleteVertexArrays(1, &m_vao);
+        m_vao = 0;
+    }
+}
+
+void OpenGLVertexArray::Bind() const {
+    glBindVertexArray(m_vao);
+}
+
+void OpenGLVertexArray::Unbind() const {
+    glBindVertexArray(0);
+}
+
+} // namespace RHI
+} // namespace XCEngine