 how to convert from clip space to pixels
      gl.viewport(0, 0, targetTextureWidth, targetTextureHeight);

      // Clear the canvas AND the depth buffer.
      gl.clearColor(0, 0, 1, 1);   // clear to blue
      gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);

      const aspect = targetTextureWidth / targetTextureHeight;
      drawCube(aspect);
    }

    {
      // render to the canvas
      gl.bindFramebuffer(gl.FRAMEBUFFER, null);

      // render the cube with the texture we just rendered to
  