anchor.diff:  
           // fragment shader code
           // TODO: 3D: figure out how to apply layout(location ...) to uniform.
  -        let samplerFragmentShaderStr =
  +        let fragmentShaderStr =
               [Constants.Render.GlslVersionPragma
                "uniform sampler2D tex;"
                "uniform vec4 color;"
  
positive.code:     /// Create a shader from vertex and fragment code strings.
      let CreateShaderFromStrs (vertexShaderStr, fragmentShaderStr) =
  
          // construct gl program
          let program = OpenGL.Gl.CreateProgram ()
          Assert ()
  
          // add vertex shader
          let vertexShader = OpenGL.Gl.CreateShader OpenGL.ShaderType.VertexShader
          OpenGL.Gl.ShaderSource (vertexShader, [|vertexShaderStr|], null)
          OpenGL.Gl.CompileShader vertexShader
          OpenGL.Gl.AttachShader (program, vertexShader)
          Assert ()
  
          // add fragement shader to program
          let fragmentShader = OpenGL.Gl.CreateShader OpenGL.ShaderType.FragmentShader
          OpenGL.Gl.ShaderSource (fragmentShader, [|fragmentShaderStr|], null)
          OpenGL.Gl.CompileShader fragmentShader
          OpenGL.Gl.AttachShader (program, fragmentShader)
          Assert ()
  
          // link program
          OpenGL.Gl.LinkProgram program
          program
  
      /// Create a sprite shader with uniforms:
      ///     0: sampler2D tex
  
negative.code:              "}"] |> String.join "\n"
  
          // create shader
          let shader = CreateShaderFromStrs (vertexShaderStr, fragmentShaderStr)
          let colorUniform = OpenGL.Gl.GetUniformLocation (shader, "color")
          let modelViewProjectionUniform = OpenGL.Gl.GetUniformLocation (shader, "modelViewProjection")
          let texUniform = OpenGL.Gl.GetUniformLocation (shader, "tex")
  