into a program
  const program = webglUtils.createProgramFromSources(gl,
      [vertexShaderSource, fragmentShaderSource]);

  const positionLoc = gl.getAttribLocation(program, 'a_position');
  const colorLoc = gl.getUniformLocation(program, 'color');
  const matrixLoc = gl.getUniformLocation(program, 'matrix');

  // Create a vertex array object (attribute state)
  const vao = gl.createVertexArray();

  // and make it the one we're currently working with
  gl.bindVertexArray(vao);

  const positionBuffer =