ec4 position;
  attribute vec2 texcoord;
  uniform mat4 u_matrix;
  varying vec2 v_texcoord;
  void main() {
    gl_Position = u_matrix * position;
    v_texcoord = texcoord;
  }
  `;
  
  const fs = `
  precision mediump float;
  varying vec2 v_texcoord;
  uniform sampler2D u_tex;
  void main() {
    gl_FragColor = texture2D(u_tex, v_texcoord);
  }
  `;
  
  // compile shaders, link program, look up locations
  const programInfo = twgl.createProgramInfo(gl, [vs, fs]);

  // gl.createBuffer, gl.bufferData f