render updates from parameter change using raf

requestAnimationFrame, that is.

glsl/raymarching.js

@@ -274,7 +274,7 @@ void main() {
  
  var sliders = findSliders(fragmentShaderSrc);
  initSliders(gl, program, sliders, function(ev) {
    render();
    requestAnimationFrame(render);
  });
  
  addSliders(sidebarEl, sliders);