merge vertexbuffer and indexbuffer to buffer.cpp/h file

Sandbox/Sandbox/Layer/DemoLayer.h

@@ -21,6 +21,7 @@ public:
 
 private:
     float m_ClearColor[4] = { 0.2f, 0.2f, 0.2f, 1.0f };
+    glm::vec4 m_TriangleColor = { 0.4f, 0.5f, 0.6f, 1.0f };
 
     std::unique_ptr<Prism::VertexBuffer> m_VertexBuffer;
     std::unique_ptr<Prism::IndexBuffer> m_IndexBuffer;