XRWebGLSubImage.imageIndex

const xrGlBinding = new XRWebGLBinding(xrSession, gl);
const layer = xrGlBinding.createProjectionLayer({ textureType: "texture-array" });
const framebuffer = gl.createFramebuffer();

xrSession.updateRenderState({ layers: [layer] });
xrSession.requestAnimationFrame(onXRFrame);

function onXRFrame(time, xrFrame) {
  xrSession.requestAnimationFrame(onXRFrame);

  gl.bindFramebuffer(gl.FRAMEBUFFER, framebuffer);
  let viewport = xrGlBinding.getSubImage(layer, xrFrame).viewport;
  gl.viewport(viewport.x, viewport.y, viewport.width, viewport.height);

  for (let view in xrViewerPose.views) {
    let subImage = xrGlBinding.getViewSubImage(layer, view);
    gl.framebufferTextureLayer(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0,
      subImage.colorTexture, 0, subImage.imageIndex);
    gl.framebufferTextureLayer(gl.FRAMEBUFFER, gl.DEPTH_ATTACHMENT,
      subImage.depthStencilTexture, 0, subImage.imageIndex);

    // Render from the viewpoint of xrView
  }
}