Core.Graphics: Make GraphicsController GAPI-independent
Core.Graphics.OpenGL: Implement class of OpenGL resource handle
Core.Graphics.OpenGL: Use Handle in VertexBuffer and IndexBuffer
Core.Graphics.OpenGL: Use Handle in Shader
Core.Graphics.OpenGL: Make constructors consistent with D3D11
Core.Graphics.D3D11: Implement method Renderer::Clear
Fractal Generator: Add alpha channel to PNG images
Live Wallpaper: Set render target to back buffer