support continuous animation/playing and iGlobalTime

interesting aside: `uniforms` are only availlable if they are actually
*used* in the program, i.e. it isn't enough to just declare them, they
must really be used somewhere.

glsl/raymarching.js

  var fragmentShaderSrc = `precision highp float;
uniform float iGlobalTime;
uniform vec2 iResolution;
uniform vec3 iMouse;
float DistanceEstimator(vec3 pos) {
  pMod1(pos.x, offset.x);
  pMod1(pos.y, offset.y);
  pMod1(pos.z, offset.z);
  pMod1(pos.z, offset.z + sin(iGlobalTime));
  return sphere(pos);
}
      gl.drawArrays(gl.TRIANGLES, 0, 6);
    };
    
    tt.draw = function() {
      requestAnimationFrame(draw);
      render();
    tt.playing = false;
    tt.draw = function(timestamp) {
      if (tt.playing) {
        requestAnimationFrame(tt.draw);
      }
      gl.uniform1f(iGlobalTime, timestamp / 1000);
      tt.render();
    }
    
    tt.playingControls = document.createElement("div");
    var playPauseButton = document.createElement("button");
    playPauseButton.textContent = "Play";
    playPauseButton.onclick = function(ev) {
      tt.togglePlaying();
    }
    tt.playingControls.appendChild(playPauseButton);
    
    tt.togglePlaying = function() {
      if (!tt.playing) {
        tt.playing = true;
        tt.draw();
      } else {
        tt.playing = false;
      }
      playPauseButton.textContent = tt.playing ? "Pause" : "Play";
    }
    
    window.addEventListener('keydown', function(ev) {
      if (ev.keyCode == 32 && document.activeElement != editor.el) { // Space
        ev.preventDefault();
        tt.togglePlaying();
      }
    });
    
    tt.canvas.addEventListener("mousemove", function(ev) {
      gl.uniform3f(iMouse, ev.mouseX, ev.mouseY, 0.0);
    });
  sidebarEl.id = "sidebar";
  document.body.appendChild(sidebarEl);
  
  var sliders = findSliders(fragmentShaderSrc);
  var sliders = findSliders(tt.fragmentShaderSrc);
  initSliders(tt.gl, tt.program, sliders, function(ev) {
    requestAnimationFrame(tt.render);
  });
  
  addSliders(sidebarEl, sliders);
  
  sidebarEl.appendChild(tt.playingControls);
  
  tt.render();
  
  var editor = {};
        sidebarEl.innerHTML = "";
        sliders = findSliders(tt.fragmentShaderSrc);
        initSliders(tt.gl, tt.program, sliders, function(ev) {
          requestAnimationFrame(tt.render);
          // rerender if not in animation mode (otherwise `tt.draw` will already do that)
          if (!tt.playing) {
            requestAnimationFrame(tt.render);
          }
        });
        addSliders(sidebarEl, sliders);
        
        sidebarEl.appendChild(tt.playingControls);
        tt.render();
        clearError();